1 INTERCALIBRATION OF PHOTOMULTIPLIER TUBE TEST BENCHESTHROUGH PRECISION TESTING OF THIER INTERNAL PHOTODIODES Barry S. Spurlock The University of Texas at Arlington Abstract The tile calorimeters of the ATLAS detector of the Large Hadron Collider at CERN will require over ten thousand photomultiplier tubes. These photomultiplier tubes will be subjected to rigorous testing in order to ensure that they fulfill their operational requirements before installation in the detector. Testing such a large volume of photo-multiplier tubes on schedule has necessitated the construction of a number of test benches. These tests are performed by comparing the performance of photomultiplier tubes to that of a large-area photodiode. Intercalibration of these test benches is required to ensure that the test results are consistent at all test sites. This will be accomplished through precise testing of the photodiodes used in each of the test benches. This necessitated the design and construction of a unit capable of performing the precision testing of these large-area photodiodes.
Transcript
1
INTERCALIBRATION OF PHOTOMULTIPLIER TUBE TEST BENCHESTHROUGH PRECISION TESTING OF
THIER INTERNAL PHOTODIODES
Barry S. Spurlock The University of Texas at Arlington
Abstract
The tile calorimeters of the ATLAS detector of the Large Hadron Collider at CERN will require over ten thousand photomultiplier tubes. These photomultiplier tubes will be subjected to rigorous testing in order to ensure that they fulfill their operational requirements before installation in the detector. Testing such a large volume of photo-multiplier tubes on schedule has necessitated the construction of a number of test benches. These tests are performed by comparing the performance of photomultiplier tubes to that of a large-area photodiode. Intercalibration of these test benches is required to ensure that the test results are consistent at all test sites. This will be accomplished through precise testing of the photodiodes used in each of the test benches. This necessitated the design and construction of a unit capable of performing the precision testing of these large-area photodiodes.
2.1. Main Chassis ............................................................................................... 5 2.2. Central Shaft................................................................................................ 5 2.3. Source Cap .................................................................................................. 5
2.3.1. Filter Holder ....................................................................................... 6 2.4. LAP Cap...................................................................................................... 7 2.5. Power Supply and Power Circuitry ............................................................. 7 2.6. Connections, Controls, and Displays .......................................................... 7
Chapter 3. Prototype Electrical Systems.................................................................... 9 3.1. Power System Design.................................................................................. 10
3.1.1. Power System Connections, Switches, and Controls ......................... 11 3.1.2. Internal Power Supply ........................................................................ 11 3.1.3. Voltage Regulators ............................................................................. 11
3.2. Input Circuit Design .................................................................................... 11 3.2.1. Light Emitting Diode (LED) .............................................................. 11 3.2.2. LED Intensity Control and LAP Size Selection Control.................... 13
3.4. Circuit Analysis........................................................................................... 15 3.4.1. Analysis of Output Circuit ................................................................. 17 3.4.2. Analysis of LED/LAP Interaction ...................................................... 18 3.4.3. Analysis of Input Circuit .................................................................... 19 3.4.4. Conclusions Based on Numerical Analysis........................................ 19
Chapter 4. Viability Testing ..................................................................................... 19 4.1. Experimental Set Up ................................................................................... 19 4.2. Data from Viability Tests ............................................................................ 20 4.3. Conclusions from Viability Tests................................................................ 20
Chapter 5. Prototype Construction ........................................................................... 22 5.1. Design of Mechanical Components ............................................................ 22
5.1.1. LAP Cap............................................................................................. 22 5.1.2. LED Cap............................................................................................. 24 5.1.3. Central Shaft....................................................................................... 25 5.1.4. Main Chassis ...................................................................................... 28
5.2. Construction of Mechanical Components ................................................... 28 5.3. Selection and Purchase of Electrical Components ...................................... 28 5.4. Assembly of the Prototype .......................................................................... 28 5.5. Troubleshooting .......................................................................................... 29
5.5.2. Low Output ........................................................................................ 29 5.5.3. Internal Capacitance ........................................................................... 29 5.5.4. LAP Connection................................................................................. 30 5.5.5. Temperature Gauge ............................................................................ 30 Chapter 6. Testing the Prototype.............................................................................. 31
6.1. Testing the Light Tight Chamber ................................................................ 31 6.2. Temperature Testing.................................................................................... 31 6.3. Repetition of Photodiode Installation.......................................................... 31
6.3.1. Data from Repetition Tests ................................................................ 31 6.3.2. Data Analysis ..................................................................................... 35
6.4. Testing Operation with 220VAC Input Power............................................ 43 Chapter 7. Conclusions ............................................................................................ 43 Appendix A. Parts List ............................................................................................. 48 Appendix B. Specifications for Electronic Components ......................................... 49 Appendix C. Dimensional Outlines for Mechanical Components........................... 55 Bibliography.............................................................................................................. 98
4
1. Introduction
The ATLAS detector calorimeters will use over ten thousand photomultiplier tubes (PMTs). Due to the large number of PMTs, these will be tested at a number of locations using identical test benches, constructed for this purpose. It is important that the results from different PMT testing stations be comparable at the same level. To this end, a method of intercalibrating the test stands has been developed. The various test benches assess the quality of PMTs by comparing their response when stimulated by an assortment of light emissions to the response of a central photodiode. Consequently, we can intercalibrate the different test stands by testing their internal LAPs. Though no specific requirements on the accuracy of this intercalibration were specified in the technical design report of the ATLAS tile calorimeter, it offers a general statement that the calibration of PMTs during operation should be ~1%. Thus, our goal will be to achieve similar accuracy. There are two possible ways of achieving our goal. Either we bring the LAPs to one place for testing, or we ship the test equipment to each different location. Having the versatility to operate in either of these ways is preferable and easily obtainable. This requires that our unit must be compact enough to be easily shipped. Our prototype will operate solely in ‘DC mode’ in which the LAP is excited by a continuous, steady level of light. This will be the most simple to construct, allowing testing to commence as soon as possible. Our basic design will allow for increased capabilities to be added to future versions. One improvement is the inclusion of a pulse mode, where the LAP is excited by a short burst of light. Another possibility is the addition of an intermittent mode, where the photodiode is excited by the emissions of a radioactive source. The emissions of a radioactive source are more consistent that other sources, and this could lead to more accurate measurements. We could also include a control port, which will allow an outside computer to control all of these modes. In this thesis, we will discuss the design and construction of a unit to test LAPs in DC mode. We will present how this design achieves our goals. We will then present the results of extensive studies of the performance of the test unit.
2. Prototype Design Objectives The highest priority is reproducibility. We must be able to create the exact test conditions every time we perform a test on a LAP. This includes ensuring the precise separation and alignment between the LAPs and the light source, as well as keeping the intensity and orientation of the source exact. To this end, our parts will be rectangular rather than circular to allow for greater precision in machining.
5
Another major concern is the elimination of “crosstalk.” In other words, we must ensure that electrical signals directed to the source will have no effect upon the LAP's output signal. The impact of noise is also an issue, and care must be taken to reduce noise wherever possible. This is especially important in those locations where our signal to noise ratio is low. Since the signal coming from the LAP is very small, we must be particularly concerned with minimizing noise prior to amplification of this signal. Finally, as with all electrical systems, the appropriate measures of electrical protection are needed to minimize the possibility of damage due to shorts and surges.
2.1. Main Chassis Unlike the central shaft, the outer casing is not required to be light tight. It must be durable enough to withstand rough handling during shipping. The main chassis needs to provide a stable base for our testing, and it must have ample room to place controls and displays. It should be composed of a conducting metal, which can be grounded. This will provide shielding against the influence of external electromagnetic fields.
2.2. Central Shaft The central shaft is where the light from our source will propagate to the LAP under testing. It must be “light tight,” meaning that it no outside light can penetrate into its interior. At one end, it must connect to the source cap, which will hold the light source. At the other end, it will connect to the LAP cap, which will hold the LAP to be tested. This design of changeable source and LAP caps will allow us to use this unit with a variety of input and output sources. Care must be taken to ensure that the connections between the central shaft and the caps do not allow outside light to enter.
2.3. Source Cap The source cap will hold our light source, and it must be removable to allow replacement of that source. By making it interchangeable, we can perform a variety of tests using different light sources with minimal adjustment to the basic unit. Our design will only include a light emitting diode (LED) as a light source, but having the ability to upgrade to allow operation with a pulsed or radioactive source will provide a more complete test of the LAP characteristics. The cap must lock into position with the central shaft to ensure that the spacing between the source and the LAP is identical in every test. As mentioned earlier, this connection must also be light tight. The design will allow for the addition of a filter holder, but this may not be necessary. A general layout for our test unit is shown in figure 1.
6
Figure 1. Layout of the LAP Test Unit. 2.3.1. Filter Holder Our design will be compatible with an easy-to-construct 3-filter holder. This will allow us the option of using a combination of filters including wavelength selection, diffusion, and/or attenuation. The DC prototype will first be tested without any filters. If it is determined that filters are necessary, then the filter holder will be installed. The required filters will be installed permanently during the design and testing stages since replacement of these filters could lead to an additional source of error. If a mixer is required, it will be connected to the filter holder.
7
2.4. LAP Cap The detachable LAP cap must allow us to easily exchange LAPs. It must also lock into position on the central shaft in order to ensure definite positioning of the LAP, and create a light tight connection. The LAP will be held in precise position by a frame, which is secured to the LAP cap. Since testing will be performed upon photodiodes of various sizes, this will require a different frame for each size and shape. Each type of LAP will also have slightly different pin positioning. This requires that we adopt an electrical connection that can adapt to each type of LAP. An extra LAP cap will be constructed in which the LAP is permanently attached. This will serve as an additional control. 2.5. Power Supply and Power Circuitry There are a variety of supply voltages available from the PMT test benches themselves, but variations from one bench to another could have an effect upon our readings. Instead, our test unit will have its own internal power supply. Supplies are available that can be powered from both 110VAC and 220VAC inputs. This will allow us to perform tests anywhere there is a wall socket. Separate voltage regulators will provide power to the LED and the LAP amplifier in order to minimize crosstalk. This is a cost-effective way to supply the required voltages and it also helps to provide a degree of surge protection. The meter used to read and display the LAP output signal will require its own independent power source. Consequently, we will include one or more batteries to power the meter. Most of the circuitry can be placed anywhere inside the main casing, but the LAP amplifier should be placed as close as possible to the LAP itself within the LAP end cap. This will allow us to amplify our signal before it becomes degraded by noise. 2.6. Connections, Controls, and Displays The controls and displays are listed below. Many are visible in the perspective drawing, figure 2.
1. Main Power Switch: This switch connects or disconnects the unit from the external source of power.
2. DC Test Switch: This switch supplies power to the source LED. 3. LED Intensity Control: This adjusts the input voltage supplied to the LED in
conjunction with the LAP size selection control, and thus controls the amount of light emitted. It will have five settings allowing us to test the LAP response to a range of light intensities.
4. LAP Size Selection Control: This switch scales the LED intensity so that we can obtain readings at optimal levels for each of the three sizes of LAPs that will be tested.
5. Power Indicator Lights: These verify that the power supply is functioning. 6. Meter Display: This displays the voltage of our amplified LAP signal.
8
7. Meter Scale Switch: This adjusts the maximum voltage displayed by the meter, thereby maximizing the precision of each reading.
8. Power Input Socket: This will be used to connect the test unit to external power, namely a wall outlet.
9. Source Selection Switch: This adjusts the internal settings to accept either 110VAC or 220VAC as external power.
Figure 2. Perspective Diagram of Test Unit.
9
3. Prototype Electrical Systems The circuitry of the prototype can be broken down into three separate component systems. The first will be called the “power system,” and it will include the power supply and its controls/displays. The second part will be called the “input circuit,” and it will contain all of the components needed to produce the illumination required for the test (up to and including the LED). The third part will be called the “output circuit” and it will contain all of the components required to collect the light and display our measurement (including the LAP and the display). The output circuit is essentially a light meter. The general operation is shown by the flow chart shown in figure 3.
Figure 3. Operational Flow Chart.
10
3.1. Power System Design The power system will deliver electric power to every part of the unit with one exception; the battery used as an independent supply to the digital panel meter will be included in the output circuit. We intend to draw our external power from a standard wall outlet (either 110VAC or 220VAC), and deliver this to our internal power supply. This will be done through a standard (IEC) computer cable and socket. The input will be grounded to the main chassis, and fuses will be used to protect the power supply and other components. The internal power supply will then provide ±15VDC for operation of the components of the tester. LEDs will be used as power indicators, with one accepting power from the +15VDC source and the other accepting power from the -15VDC source.
The power supply output will then be further split into three voltage regulators. The first will be called the input voltage regulator, and it will provide the voltage used in the input circuit (+5VDC). The other two will provide the voltages needed for our amplifier circuit (+12VDC and –12VDC). The power system will also include the power switch and the power indicator light(s). A schematic is shown in figure 4.
Figure 4. Schematic Diagram of Power System.
11
3.1.1. Power System Connections, Switches, and Controls A simple rocker switch will be used as our power switch. Two inexpensive LEDs will act as indicators, one for the positive voltage output of the power supply and one for the negative. To draw power from a wall socket, an ideal choice for the power input socket is an IEC female power connector. This would enable us to use the standard power cords used to power computers. These cords should be readily available if some problem should arise. A source selector switch will be required to choose between 110VAC and 220VAC. This switch must also make the desired changes to the input connections of the power supply (so that it can accept the appropriate input voltage), and divert the input current through the correct fuse. The DC test switch will be placed prior to the voltage regulator used for the input circuit. A toggle switch seems a very functional choice.
3.1.2. Internal Power Supply Our internal power supply must take the input power, 110/220V@60Hz, and provide power needed for our voltage regulators. All of our voltage regulators can be powered by a supply that can provide both +15VDC and –15VDC. It was decided to use a linear supply in order to minimize noise. The supply chosen was an IHAD15-0.4 from International Power. It is small and relatively light, with good regulation. One concern about this supply was its low power capacity (it can only produce 0.4A through either channel), but estimations of our needs indicate that this is more than adequate.
3.1.3. Voltage Regulators The input voltage regulator will take +15VDC input, and supply 5VDC to the LED via the test selection switches (and resistors). The output voltage regulators will provide +12VDC and –12VDC to the amplifier circuit. All are inexpensive and readily available. The input voltage regulator will be connected to the DC test switch.
3.2. Input Circuit Design The purpose of the input circuit is to produce the illumination needed to conduct our test. It will consist of the LED, the LED intensity control, and the LAP size selection control. The input circuit is shown in figure 5. This figure also includes the input voltage regulator, which is part of the power system, in order to show the connections. 3.2.1. Light Emitting Diode The LED being used is an NSPB310, which was in our possession at the beginning of this project. The light emitted from the LED has a spectrum that can be approximated by a Gaussian distribution with a peak at 465nm and a width of 30nm. The peak frequency is
12
close enough to 480nm to suit our needs, with a nominal intensity of half a candela. The specifications for this LED can be found in appendix B.
Figure 5. Schematic Diagram of Input Circuit. We can approximate the LED intensity with the equation shown. The leading coefficient, I0, will be dependent upon the voltage applied to the LED:
13
In order to determine the total power emitted by the LED we must then integrate over all possible wavelengths:
The fraction of this luminous power that reaches the photodiode will be determined by the geometry of our experimental layout.
3.2.2. LED Intensity Control and LAP Size Selection Control The function of both of these controls is to determine the amount of current sent to the LED. Our goal is to produce a LAP output voltage near 5VDC for the high setting of the LED intensity control and a LAP output voltage near 0.1VDC for the low setting of the LED intensity control. It should produce these results for each size of LAP being tested. This is complicated by the LED’s non-linear relationship between the input current and intensity. In order to produce the desired results, we will combine the LED intensity control with the LAP size selection control in the same configuration as a digital multiplexer. Each of the fifteen possible settings of these two controls will divert the current through a resistor unique to that setting. The resistor chosen will restrict the flow of current to the LED to produce the required light intensity. The values of the resistors were determined by experiment using a control photodiode. The original design neglected to include the diodes in series with each resistor. Failure to include these allowed the current to flow back through many different paths, increasing the current delivered to the LED.
3.3. Output Circuit Design The purpose of the output circuit is to measure the light produced by the input circuit and display the results. It will consist of the photodiode, the photodiode amplifier circuit, and the meter/display. It is essentially a powerful light meter. A schematic of the output circuit is shown in figure 6. 3.3.1. Large Area Photodiodes The photodiodes to be tested will be Hamamatsu model numbers S3590-03, S2744-03, and S6337-01. Their active surface areas are 100 sq. mm, 200 sq. mm, and 400 sq. mm, respectively. These photodiodes have shunt resistances which are greater than 1MΩ (as we will see, an exact figure is unnecessary), and generate an output current of 0.26A per watt of incident light at 480nm. This is a slight approximation since our chosen LED will emit light centered on 465nm. The equivalent circuit for these photodiodes can be seen in figure 8.
14
Figure 6. Schematic Diagram of Output Circuit. 3.3.2. Amplifier Circuit The amplifier must take a very small current from the photodiode, and from it generate a significant voltage that can be measured by the voltmeter. The logical choice for an amplifier circuit is to use the same circuit that is used in the PMT test stand. This circuit is shown in figure 7. The op amps used are TL083s with an input resistance of 1012Ω and a differential voltage gain of 2x105. For DC operation, we can safely ignore the feedback capacitor of the first op amp, but the 4MΩ feedback resistor will prove very significant. The second op amp is merely a voltage follower used to isolate the amplifier from the output. This is a low noise circuit, but noise can still affect our system prior to amplification (where the signal is very small). Thus it is important to keep the distance between the LAP and the amplifier as small as possible. An equivalent for this circuit can
15
be seen in figure 8, which shows the combination of amplifier and photodiode equivalents.
Figure 7. Amplifier Circuit.
Figure 8. Equivalent Circuit for Output Circuit.
3.3.3. Voltmeter/Display A number of suitable digital panel meters are available commercially. These include both the logic and the display and have high accuracy. The meter we have selected is powered by 9VDC and is accurate to 0.05%. Specifications can be found in appendix B.
3.4. Circuit Analysis In this analysis, we will omit for greater clarity. All values will assume SI standards for units. Thus, currents will be in amperes, all voltages will be in volts, and all resistances will be in ohms. The complete circuit is shown in figure 9.
16
Figure 9. Schematic Diagram of Complete Circuit.
17
3.4.1. Analysis of Output Circuit For analysis purposes, the only components of the output circuit that concern us are the LAP and the DC amplifier circuit. An equivalent circuit is shown in figure 8. Our starting point is the current equation at node A:
IL = ID + IR + IO – IF We will consider each of the currents separately. The photodiode specifications indicate that we can treat IL as a current source which is dependent upon the power of the incident light, PI (in watts), that strikes the surface of the photodiode:
IL = 0.26⋅⋅⋅⋅PI
We will assume that the current through the diode obeys the standard equation for diodes at room temperature. The equation relates the diodes current, ID, to the voltage across it, VP, and a constant called the saturation current, IS:
ID = IS⋅⋅⋅⋅exp[VP/26mV]-1
The resistor current, IR, the op amp input current, IO, and the feedback current, IF, are given by Ohm’s law:
Our current equation can now be simplified. Our first simplification comes from noting that IR and IO are insignificant compared to IF (Since R is on the order of MΩ and the input resistance of the op amp is 1012Ω). Next, we can simplify our diode current. Since we expect an output voltage on the order of 1V, VP must be on the order of 10µV. Consequently, VP/26mV is much less than 1, and we can expand the exponential:
exp[VP/26mV] ~ 1 + VP/26mV
ID = IS⋅⋅⋅⋅exp[VP/26mV]-1 ~ IS⋅⋅⋅⋅VP/26mV
When we consider that IS is usually on the order of nanoamperes, we find that ID is also insignificant compared to IF. Thus we obtain a simple current equation:
18
IL = – IF Now we substitute the values of these currents:
0.26⋅⋅⋅⋅PI = VP/20
Finally, we obtain VO in terms of PI:
VO = 2x105⋅⋅⋅⋅VP = 2x105⋅⋅⋅⋅5.2⋅⋅⋅⋅PI = 1.04x106⋅⋅⋅⋅PI
3.4.2. Analysis of LED/LAP Interaction The power of light incident on the photodiode (PI) is dependent upon the intensity (I0) of the LED, the area (A) of the photodiode, and the distance (d) between the LED and the photodiode. The intensity of the LED will be given in candelas (cd). To keep things simple, we will use the approximation that the distance between the LED and every part of the photodiode is the same. We will use the result shown earlier in section 3.2.1 for the combined intensity emitted by the LED, which relates the total power emitted by the LED, ITotal, to the power emitted at the peak frequency, I0:
We can determine the power that reaches the LAP from simple geometry:
Here, “A” is the active area of the LAP, and “d” is the distance between LED and LAP. This ignores associated with adding filters in the light's path. It also ignores additional losses that might be caused by the directivity of the LED where the power is not uniformly distributed across the face of the LAP. We can now combine this result with the formula from section 3.4.1 to obtain a value for VO:
3.4.3. Analysis of Input Circuit
19
At this point, all we need is to determine the LED intensity (I0) in terms of the voltage applied to the LED. Unfortunately, this relationship is non-linear. In addition, the specifications we want are given as graphs rather than equations or equivalent components, and this further complicates matters. A quick order of magnitude estimate will shed some light on the situation. Assuming that A and d2 are of the same magnitude simplifies things dramatically. We discover that to produce an output voltage on the order of 1VDC, we need an intensity of 1mcd (millicandela). In order to produce the low light levels required, the current supplied to the LED will be very small. This region of operation is not shown in the product specifications. This limits what may be accomplished with calculations, and forces us to rely upon experimentation to obtain the desired operating voltages and currents.
3.4.4. Conclusions Based on Numerical Analysis The primary factors that will determine the output for a given LED are the intensity of the LED and the LED/LAP separation. The LED intensity will be controlled through the choice of resistors in our circuit, and these values will be refined through further experimentation. The LED will be operating at very low current levels to produce the values needed for our tests. The LED/LAP separation will be a fixed parameter, and once it is built into the design it will not change. Thus, the choice for this separation is a very important element that we need to determine through viability tests. 4. Viability Testing
4.1. Experimental Set Up Our goal at this stage was to ensure that the LED would operate normally in the region we expect it function, and to ensure that there were no major surprises waiting for us. To this end we set up an extremely crude version of our apparatus. A variable resistor was placed in series with a standard power supply in order to create a variable power supply. This was used to supply power to our LED, while a meter was attached to measure the voltage across the LED. This measurement is what we have called the input voltage. The S3590-03 LAP, which has an active surface that is 10mm on a side, was attached to an amplifier circuit. This pair was then laid in the light tight box facing the LED. A second power supply was used to power the amplifier, and its output was run to a second voltage meter. This voltage is what we have called the output voltage. The LAP/LED separation (d) was set using a ruler. The alignment of the two components was made by visual inspection while the components lay on their sides in the box. The LAP and LED were not secured in these positions. 4.2. Data from Viability Tests
20
We took measurements for separations of 5cm, 7.5cm, 10cm, and 12.5cm. For each distance the input voltage was varied from 2.4VDC (the lowest our setup would allow), and increased until our output voltage reached the level of amplifier saturation (around 9.7VDC). The results from this test are shown in table 1, and this data is also presented as a plot in figure 10.
Table 1. Data from Viability Test
This same data was used to make a second plot, which is shown in figure 10. A few additional measurements were made at 10cm separation to complete this chart. On this graph the output voltage is plotted against the inverse square of the distance of separation. Our expectation was that this would produce nice linear results. Instead we found a distinct curvature to the lines. We believe that this curvature is a direct result of effects occurring at the edges of the LAP. In the analysis we assumed that the distance from the LED to every point on the LAP surface was the same, and the validity of this assumption grows weaker as the separation shortens. Another effect is caused by the directivity of the LED. As the separation grows smaller, the area of the LAP begins to extend outside the region most strongly illuminated by the LED. This causes the voltage to become nonlinear as the separation grows smaller. 4.3. Conclusions from Viability Testing The viability tests provided useful insights. First and foremost, it has shown that the basic concept is sound. The photodiode/amplifier combination is very sensitive and requires little illumination to produce useful results. This opens the possibility of using a weaker (and hopefully less expensive LED). As the separation nears 5cm, we begin to concentrate power upon the center of the LAP, resulting in weaker testing of the edges and corners of the LAP. We can compensate for this effect with a diffusion filter (or a
21
mixer) or by changing to an LED with a broader illumination. Of course, this effect also disappears by increasing the LED/LAP separation. We have decided to operate at a separation of 5cm. At this range the separation is large enough that fringe effects are still minimal. It is also large enough to allow for the insertion of a filter holder and filters. At this range, the LED is strong enough that any losses in such filters will pose no serious problem.
Figure 10. Voltage Gain with LAP/LED Separation.
22
Figure 11. Output Voltage vs. LAP/LED Separation. 5. Prototype Construction
5.1. Design of Mechanical Components The design of the mechanical components began with the LAP and proceeded outward. The LAP cap was designed around the photodiode. Next, the LED cap was made to match up with the LAP Cap, and then the central shaft was designed to hold both caps. Finally, the main chassis was designed around the central shaft.
5.1.1. LAP Cap Checking the dimensional outlines of the various LAPs gave us the minimum size required for the LAP frames, which are designed to hold the LAPs in precise position. In order to accommodate three different sizes of photodiodes we need three different frames. The three frames were made with the same outer dimensions and placement of retaining
23
screw holes, making them interchangeable. The mounting surface was recessed to allow the LAP frames to be positioned correctly. Making our cap function with any of the three sizes of LAPs created another problem. The placement of the pins is different for each size of photodiode. The different pin placements were confined to a small enough space that using separate connections for each LAP size would have been problematic. Instead we elected to make the connections by hand, after the LAP and frame had already been attached to the cap. Since the separation between the two pins was the same for each size, a two-pin connector was used to make the connection. A second layer, to which the amplifier would be attached, was placed a short distance behind the first. Placing the amplifier as close to the LAP as possible will minimize the effects of noise. Any noise entering the system prior to amplification would be amplified along with our signal. Despite this, enough space was kept in between so that we could make our manual connections. Both layers are held in place inside a rectangular tube. This tube would fit snugly inside the central shaft. The overlap of end caps and central shaft would help to eliminate any possibility of light passing in between and reaching the central shaft. Rails were allowed to extend along opposite sides with the intention of having them slide into grooves inside the central shaft. This would help us to precisely position the LAP cap. A retaining pin was designed to pass through both tubes, the LAP cap, and the central shaft, as another aid to LAP cap positioning. A plate was installed inside the LAP cap to isolate the retaining pin holes from that end of the light tight chamber. A second pair of holes were added, one to allow us to bring in power to the amplifier and one to take our signal out. We elected to make external connections so that we can ensure good electrical connections. Our initial intention was to fill these holes after our signal and power lines were passed through, but it turns out that these holes do not degrade the light-tight nature of our central shaft. A plate was placed over the rear of the cap to seal it, and a rim was left extended around it to provide something to grip when removing the cap. The interior surface was painted flat black to minimize reflection. Finally, decals were added to identify the lines leaving the LAP cap and ensure its proper orientation. A scale drawing of the LAP cap is shown in figure 12.
24
Figure 12. Scale Drawing of the LAP Cap.
5.1.2. LED Cap The LED cap was designed to use the same rectangular tube shape as the LAP cap. In this manner, it would slide into the same grooves from the other side of the shaft. It was left shorter since there was less need for space. A single plate acts as a mounting surface for the LED, and it will also act as a mounting surface for a filter holder should one be added. The LED mount securely holds the LED in place. The line that carries power to the LED mount passes through a hole in the side. As with the LAP cap, a retaining pin holds the LED cap in place, and a plate covers the back side of the LED cap with a rim left as a grip. In order to minimize reflections, the interior surface was painted flat black. Decals were added to identify the line leaving the LED cap and to ensure that it is oriented correctly. A scale drawing of the LED cap is shown in figure 13.
25
Figure 13. Scale Drawing of the LED Cap.
5.1.3. Central Shaft The central shaft was designed as a rectangular tube, large enough so that both the LED and the LAP caps would fit inside. Grooves were positioned to align with the rails on the caps, allowing the caps to be precisely positioned. Inside the shaft, a rectangular “stop” was placed on opposite sides. This would catch both the LED and the LAP cap, and ensure that they were appropriately separated. Extensions were left on either end to allow the retaining pins of each cap to pass through. A scale drawing of the central shaft is shown in figure 14. A similar drawing showing how the LED and LAP caps connect is shown in figure 15.
26
Figure 14. Scale Drawing of the Central Shaft.
27
Figure 15. Scale Drawing of the Central Shaft with Caps Attached.
28
5.1.4. Main Chassis The primary consideration for the size of the main chassis was the size of the components that needed to be within it. The width was designed to ensure that the installed LAP and LED caps would extend no further than the edge of the main chassis. The height was chosen to leave extra space below the shaft to allow wires to pass beneath. The depth was chosen to allow room for the power supply behind the central shaft and room for the controls on the front panel. The front panel was designed to allow installation of all of the controls and displays, with the exception of the input power selection switch, which is located on the back panel. The power input and fuses were also placed on the back panel. Holes were also added to the back panel to vent any excess heat generated by the power supply. Decals were added to identify the electrical connections on the surface, as well as identifying all of the controls and displays, and rubber feet were added to the bottom surface to keep the metal box from destroying the surfaces upon which it sits. We named the prototype LAPLACE, which stands for “large area photodiode luminance analysis calibration equipment.” A perspective drawing made to scale from the dimensional outlines for the components was shown in figure 2. 5.2. Construction of Mechanical Components The LED and LAP caps, the main chassis, and the central shaft were all machined to a tolerance of one thousandth of a inch in the machine shop of the UTA physics department by Jimmy Hanhart utilizing a Trump B6EC. Wallace Lutes and I performed additional machining.
5.3. Selection and Purchase of Electrical Components Major electrical components, [and by this I mean the expensive components] were selected by comparison shopping. Minor components were added to these orders to keep shipping costs to a minimum. A list of the electrical components and the suppliers of those components are shown in appendix A.
5.4. Assembly of the Prototype I assembled the prototype in the UTA high energy physics electronics lab. Armen Vartepatian assisted me by soldering the most difficult connections. Onder Anilturk gave me additional support by performing some soldering while keeping me company in the lab late at night.
29
5.5. Troubleshooting A number of difficulties arose during the construction of this prototype. What follows is a short description of each problem and how we resolved them.
5.5.1. Readout Fluctuation The first problem encountered were small, random fluctuations in the readout. Initially it was thought that the cause might be the absence of a load resistor. A one mega-ohm load resistor was installed, but this did not solve the problem. We discovered that the problem occurred in the power provided to the digital panel meter. Originally we had supplied power to the digital panel meter via a dedicated voltage regulator, which received its power from the unit’s power supply. Unfortunately, the meter required an independent source, completely isolated from the signal’s power supply. This was solved by adding a 9V battery with the sole purpose of providing power to the meter. The previous diagrams include this modification.
5.5.2. Low Output The next difficulty was that the meter’s readings were significantly different from our expectations. The resistors selected by the intensity control and the LAP size selection control were chosen to achieve the appropriate output levels for each setting. When we tested photodiodes, our reading were less than half of what we had intended. We found that the original multiplexer design allowed current to flow through more than one resistor by flowing back through other resistors. This was solved by the installation of diodes in series with each resistor in order to prevent these currents. These diodes are shown in the previous diagrams. Addition of the diodes forced us to change the values of all of these resistors. The output values still differ from our desired levels, and we believe that the new resistor values are at fault. It was decided that correcting these resistors would be postponed until a later date since the output range is adequate for our measurements.
5.5.3. Internal Capacitance Our next hurdle was a capacitance problem. When testing the stability of our measurements with time we discovered that the output values would slowly fall. After taking readings for two weeks we found that this decay was very close to an exponential function with a time constant of about 6 days. This data is shown in figure 16. The large time constant could only occur with a large value of resistance. Consequently, we examined the circuit around the largest resistors. It was found that the connection between the one mega-ohm load resistor and ground was loose. At this time we realized that the amplifier circuit has a built-in ten kilo-ohm load resistor, making the one mega-
30
ohm load resistor redundant. We removed the redundant resistor, and this solved the problem.
Figure 16. Exponential Decay of Output Voltage.
5.5.4. LAP Connection The connector initially used to connect to the LAP was designed to slide onto the ends of the pair of photodiode pins. We found that the fragile pins were bending during insertion of the LAP. It was obvious that repeated bending would eventually break the pins. To avoid this, new test leads were installed. The new connectors attach individually to each of the pins, and these connections are made by hand. These new connections do not damage the LAP pins.
5.5.5. Temperature Gauge In order to determine if temperature would have a significant effect upon testing, the unit was placed inside a refrigerator in order to cool it before measurements were taken. A significant change in our readings was discovered. In order to compensate for these temperature effects, a panel mount thermometer was purchased and installed on the unit. Once the temperature dependence is determined, we can make corrections to the test data.
31
6. Testing the Prototype 6.1. Testing Light Tight Chamber In order to ensure that the light-tight chamber would not allow any light to enter, the dark current was measured while a 500W halogen light was directed at the unit. Measurements were taken with the light at various orientations. In each case, there was no change in the output voltage measurement. We can conclude that even the brightest of external lights will have no effect upon our measurements.
6.2. Temperature Testing We subjected the test unit to temperatures over a range from 9˚C to 30˚C (48˚F to 86˚F). The low temperatures were obtained by placing the unit in a refrigerator. Two sets of data were taken under these conditions. The data was taken with no illumination from the LED in addition to all five intensity levels. To obtain the readings at the higher temperatures, the unit was placed near a heat lamp. An additional set of data was taken under these conditions. The three sets of data were mapped on the same graphs with one graph for each intensity setting. An examination of these graphs, figures 17 through 22, shows unusual behavior. It was hoped that we could make a linear approximation that would reasonably fit this data, but the behavior precludes this simple type of correction. Instead, we will use a table of multipliers generated from the data. We will use one multiplier for each even value of temperature, and we will interpolate for decimal values. We will use 22˚C (72˚F) as our base temperature, giving no correction (a multiplier of 1). These multipliers are shown in table 2. 6.3. Repetition of Photodiode Installation In this test, we took measurements for each of eight different LAPs. After measurements were taken at each of the six possible illumination levels, the photodiode was removed and replaced with a different LAP. This ensured that each photodiode was completely reinstalled after each measurement. To vary the test conditions as much as possible, measurements were taken in three separate sessions on three separate days in two different locations. Nine measurements were taken for each photodiode. 6.3.1. Data from Repetition tests The data from repetition testing is shown in tables 3 and 4. It has been sorted by photodiode to allow comparison of the results. The column on the left identifies the photodiode under test. Those that begin with ‘S’ are the smallest photodiodes, S3590-03, which are numbered consecutively. Those that begin with ‘M’ are the intermediate sized photodiodes, S2744-03. Those that begin with ‘L’ are the largest of the LAPs, S6337-01.
32
Figure17. Output Voltage Temperature Dependence for Dark Current.
Figure 18. Output Voltage Temperature Dependence for Setting 1.
33
Figure 19. Output Voltage Temperature Dependence for Setting 2.
Figure 20. Output Voltage Temperature Dependence for Setting 3.
34
Figure 21. Output Voltage Temperature Dependence for Setting 4.
Figure 22. Output Voltage Temperature Dependence for Setting 5.
35
Table 2. Temperature Corrections
The next two columns show the session and temperature, respectively. Beside these are six columns of data, one for each possible intensity setting and one with no illumination. Each photodiode has nine rows of data beside it. The data in each row was taken consecutively, and there was at least a half hour delay before the next set of measurements were taken on that photodiode.
6.3.2. Data Analysis Compilation of the data prior to temperature corrections shows less than desirable results. This is shown in table 5. We expect the best results to come from the highest illumination setting due to the fact that we will have the highest signal to noise ratio prior to amplification. Consequently, we will focus our attention on measurements taken at the highest setting. The high intensity data shows an average standard deviation of 2.12%, which is larger than desired. Better results are expected when temperature corrections are taken into account. Graphs of the high intensity data are shown in figures 23 and 24. Although the graphs in figures 23 and 24 drift over a large range, the curves seem to follow the same trends, rising and falling together. In addition, the data points for each session, separated by the dashed lines, seems to be grouped together. These general trends could be the result of some aspect of the environment. Temperature corrections were included for the compilation of data in table 6. The high intensity data now shows an average standard deviation of 1.89%. This is an improvement over the data taken without correction, but still falls short of our goals. Graphs of the temperature-corrected data are shown in figures 25 and 26.
36
Table 3. Data from Repetition Testing of the Small LAPs
37
Table 4. Data from Repetition Testing of the Large LAPs
38
Table 5. Compilation of Data without Temperature Corrections
Figure 23. High Intensity Measurements of Small LAPs.
39
Figure 24. High Intensity Measurements of Large LAPs.
Table 6. Compilation of Data after Temperature Corrections
40
Figure 25. Temperature Corrected Data for Small LAPs.
Figure 26. Temperature Corrected Data for Large LAPs.
41
The temperature correction seems to work well for the small photodiodes. Though the first two points, the data from the first session, do not match up with the other data points, they agree with each other. This may indicate that another environmental factor is affecting our data. The temperature corrected data for the large photodiodes, however, seems to be worse than the uncorrected data. This indicates that the temperature correction, derived from and used successfully on the small photodiodes, cannot be applied to the data for the medium and large LAPs. It appears that the data taken in each session was more closely related than the overall standard deviation. A compilation of the data by session is shown in tables 7 and 8. When the data is separated out on a session to session basis, the high intensity data has an average standard deviation of 0.64%. Temperature corrections were made, and this data is shown in tables 9 and 10. The high intensity data then has a standard deviation of 0.69%. It should be noted that the data improves significantly for the small photodiodes, and the standard deviation is approximately 0.40%. The same correction applied to the larger photodiodes makes the data worse. Once again, this indicates the need for a separate set of temperature corrections for the larger photodiodes. As suggested earlier, the correlation of the data from session to session indicates that there may be additional environmental factors that are affecting our readings. The shift in our data from one session to the next may be caused by a single factor, such as humidity for example. It may be possible to make corrections similar to those made for temperature. One possible method of taking all possible environmental factors into account is to simply compare the performance of the photodiode being tested with the performance of a control photodiode. We can approximate this to a limited degree in the data already taken by treating the first photodiode as our control. Since the temperature drifted over the measurements, temperature corrections are included. This is shown in table 11.
The standard deviation of the ratio of the high intensity readings is less than 1.00% for most of the photodiodes. It is curious that the standard deviation deteriorates as we move down the chart. This may be a result of the delay between the testing of the photodiode and the control. Since the diodes were tested in order, the further down the diode is listed, the larger the delay between its test and that of the control.
To present this data in the form of a graph, we have taken the ratio of these data points and scaled them by a factor of 3.20 in order to compare them easily with previous graphs. These graphs are shown in figures 27 and 28.
42
Table 7. Compilation of Small LAP Data by Session without Temperature Corrections
Improvement in the small LAP data is clearly evident. In the case of the large LAPs, we see some improvement, but the data is still very erratic. Once again, this indicates that we need to consider the larger diodes separately. Figure 29 shows the ratio of the large photodiode data to that of M1. This brings about a dramatic improvement in our data taken from the other two medium sized diodes. The data from the large photodiode, however, doesn’t show much improvement.
From this data, we might estimate that simultaneous testing (or near simultaneous testing) of the photodiodes and a control photodiode of the same size will produce standard deviations near 0.50%.
43
Table 7. Compilation of Large LAP Data by Session without Temperature Corrections
6.4. Testing Operation with 220VAC Input Power The unit was set up to accept 220VAC as its input power source and a few measurements were taken to ensure that it was operating correctly under the higher voltage.
7. Conclusions The test unit is functioning as designed, and by comparison with a control we exceed the precision we had sought. In normal operation, straight measurement of the diode response, it gives results good to 1.89%. This falls just short of the precision sought. By comparing our measurements with those of a control we can give results good to approximately 0.80%, which exceeds our expectations.
44
Table 9. Compilation of Small LAP Data by Session with Temperature Corrections There are encouraging signs that the results from normal operation can be improved. First, we saw that when the data is separated by session our results improved to within approximately 0.60%. This may indicate that other environmental factors are affecting our data. If these can be accounted for our results in normal operation should improve significantly. By increasing the high intensity output to a value closer to the maximum amplifier output, perhaps outputs of 8V or so, will increase the signal to noise ratio prior to amplification. This should create some improvement in out results.
45
Table 10. Compilation of Large LAP Data by Session with Temperature Corrections
Table 11. Average Ratio of Diode Measurements and Standard Deviation
46
Figure 27. Small LAP data adjusted by comparison with S1.
Figure 28. Large LAP data adjusted by comparison with S1.
47
Figure 29. Large LAP data adjusted by comparison with M1.
It is possible that some of our error stems from output fluctuations from the source LED. Increasing the LED’s output might shift it to a more stable level. By adding an attenuation filter we can increase the LED’s output power while maintaining the amount of light incident on the surface of the LAPs. This may also improve our results. Making separate temperature corrections for each size of photodiode will improve the testing of the larger photodiodes. Building a separate LAP cap with a permanently affixed control LAP could reduce the error obtained for comparison testing. By making all of these modifications, we should be able to improve our accuracy. It is possible that this could reduce our error into the range of 0.40%–0.50%.
48
Appendix A. Electronic Parts List
Part Name Description AMP Amplifier LAP output amplifier (UTA) BTH 9V Battery Holder Radio Shack #27-326 BTT 9V Battery Duracell 9V Battery CRD Power Cord IEC-US Power Cord (Standard Computer Cord) DPM Panel Meter 4½ Digit LCD Digital Panel Meter (MPJA#7160ME) F01 110VAC Fuse 0.50A Fuse, 5mm x 20mm F02 220VAC Fuse 0.25A Fuse, 5mm x 20mm F03 Fuse Holder (2) Bussman HTB-62M for 5mm x 20mm fuses K02 Power Selector Knob Mouser #5164-1510 K04 Intensity Control Knob Mouser #5164-1500 K05 Area Selector Knob Mouser #5164-1500 K06 Meter Scale Knob Mouser #5164-1510 LD1 Source LED Nichia #NSPB310A LD2 +15 Power Indicator Mouser #351-5004 LD3 -15 Power Indicator Mouser #351-5004 LP1 100mm2 LAP Hamamatsu #S3590-03 LP2 200mm2 LAP Hamamatsu #S2744-03 LP3 400mm2 LAP Hamamatsu #S6337-01 PS1 Power Supply International Power IHAD15-0.4 (MPJA#6638) S01 Power Switch Rocker Switch On/Off (Mouser #107DS850S-22) S02 Power Selector Switch Rotary, 2 Pos/3 Poles, Non-Short (Mouser #10WA166) S03 DC Test Switch Miniature Toggle Switch (Mouser #10TA420) S04 LED Intensity Control Rotary, 5Pos/1 Pole, Non-Short (Mouser #10WA164) S05 LAP Area Selector Rotary, 3 Pos/1 Pole, Non-Short (Mouser #10WA164) S06 Meter Scale Switch Rotary, 3 Pos/3 Pole, Non-Short (Mouser #10WA166) SKT AC Power Inlet IEC Plug (Mouser #161-0707-7-187) THM Thermometer Taylor 9940 Panel Mount Thermometer VR1 Input V Regulator +5VDC Voltage Regulator (511-L7805CV) VR2 +Output V Regulator +12VDC Voltage Regulator (511-L7812CV) VR3 -Output V Regulator -12VDC Voltage Regulator (511-L7912CV)
49
Appendix B. Specifications B.1. Large Area Photo Diode Specifications B.1.1. S3590-03 and S2744-03 Data
50
B.1.2. S3590-03, S2744-03, and S6337-01 Dimensional Outlines
51
B.2. NSPB310 Light Emitting Diode
52
53
B.3. International Power IHAD15-0.4 Power Supply
Marlon P. Jones Associates SN: 6638 Open frame linear supply Input: 110/220 VAC 47-63Hz Foldback current limit Outputs: #1: [email protected] Remote Sense #2: [email protected] No Minimum Load +/- .05% Regulation Weight: 2 lbs. 4.87”x4.00”x2.10” 123.70mm x 101.60mm x 53.34mm
B.4. Digital Panel Meter
Marlon P. Jones Associates SN: 7160ME 4½ digit LCD display ⅜” character height 200.0mVDC input 9VDC powered 0.05% accuracy Input impedance > 100MΩ Adjustable decimal point Auto polarity 2 readings per second Through panel mounting Holes on board to add scale resistors WT: 1 lb. 2⅝“ x 1¾” x ½”
54
B.5. Other Electronic Components
55
Appendix C. Dimensional Outlines
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
REFERENCES 1. P. Horowitz and W. Hill, “The Art of Electronics”, Cambridge University Press
(1980) 2. Mohammed S. Ghaussi, "Electronic Devices and Circuits: Discrete and Integrated",
Holt, Rinehart, and Winston (1985) 3. The manual of "Photomultipliers Data Handbook", Philips Corporation (1990) 4. M. Bosman et al., “Development of a Scintillator Tile Sampling Hadron Calorimeter
with Longitudinal Tile Configuration”, CERN/DRDC (1993) 5. ATLAS Technical Proposal, CERN/LHCC (1994) 6. E. Berger et al., “Construction and Performance of an Iron-Scintillator Hadron
Calorimeter with Longitudinal Tile Configuration”, CERN/LHCC (1994) 7. The manual of "Silicon Photodiodes and Charge Sensitive Amplifiers for Scintillation
Counting and High Energy Physics", Hamamatsu Corporation (1995) 8. The manual of "Photodiodes", Hamamatsu Corporation (1995) 9. Z. Ajaltouni et al., “Response of the ATLAS Tile Calorimeter Prototype to Muons”,
11. ATLAS Collaboration (Calorimetry and Data Acquisition), “Results from a
Combined Test of an Electromagnetic Liquid Argon Calorimeter with a Hadronic Scintillating Tile Calorimeter”, CERN/PPE (1996)
12. ATLAS Tilecal Collaboration, “Technical Specifications of the Test Bench for the
Test of the ATLAS Tile Calorimeter Photomultipliers”, CERN/LHCC (1996) 13. V. Reece, “ITC Submodule Assembly Handbook”, CERN (1998)
14. ATLAS Collaboration (Calorimetry and Data Acquisition), “Results from a new Combined Test of an Electromagnetic Liquid Argon Calorimeter with a Hadronic Scintillating Tile Calorimeter”, CERN/PPE (2000)
