CAUTION! This device generates high voltage direct current at safe current levels <1mA neverthless it can kick you hard if you don’t respect following precautions.
1. The high voltage produced could be still dangerous for people with an implanted pacemaker.
2. Do not operate the circuit without having it enclosed into a metallic shield. An electric discharge triggered by a finger between high voltage filtering capacitors and any other part of the circuit could result into a damage to the integrated circuits. A bridge maked by two fingers between USB connector and high voltage supply could fry your host device like PC or cell phone… DO NOT OPERATE WITHOUT A PROPER ENCLOSURE!!!
3. Do not change scintillation probe without prior disconnecting the USB cable, not doing this could trigger an overvoltage spike that could damage your board or your probe. This could create a spark capable to trigger a fire if the circuit is operated into a room saturated by an explosive air/fuel mixture.
4. High voltage capacitors need 5-15 seconds to discharge after USB cable disconnection. You are using this device at your total risk, please use common sense and respect safety regulations. I cannot verify how you are using the circuit so I cannot assume any direct or indirect, explicit or implicit responsability.
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Overview The MadExp USB PMT adapter is specifically made as power supply and pulse processing/aquisition block for building a gamma ray spectrometry system. It supply voltage to photomultiplier tube and process the pulses back from it then, the pulses are feed to the pc as audio data to be processed by MCA softwares like PRA, Becquerel Monitor and others. Main connections are:
• USB type B mini, female, for data excange with PC and 5V power to feed the board. • Stereo 3,5mm jack, female, to output raw pulses from the pulse shaper/amplifier. • BNC connector, female, here you’ll connect the coaxial cable that feeds with high voltage
your photomultiplier. The pulses are collected internally on the board so there is no need for a second cable. High voltage output is positive to ground mass.
There are two pin-headers on the board: 1. 2 pin connector marked as HV_boost, if closed with a jumper, it will rise the starting output
voltage base of the board from 500 to 1000V. The nominal output voltage range without closing it is 500V to 1250V. Closing it will change this range from 1000V base to 1450V. You can notice that in front (left side) of the high voltage filter the voltage is 150…200V higher than on the BNC connector. This is because of the filtering effect and because of filter losses. As example 1650V unfiltered from the converter will be 1450V and this is the maximum usable voltage. 99% of photomultiplier tubes don’t need more than 1200V.
2. 5 pin JTAG is for programming the firmware into the MCU processor. There is a 6 position voltage selection dip switch. A chart is provided where you can find the correct setting for your desired output voltage. Functional blocks
High voltage converter
3. Transform 5V supply from USB to 500-1450V needed by photomultiplier. 4. Output voltage is programmable via a 6 position dip switch. 5. Control output voltage via an analogic-digital control loop. It uses a 88Mohm voltage
sensing probe. 6. The MCU runs at 16Mhz reading the output voltage and stabilizing it via a 4095 point PWM
waveform and it have a soft start feature that ramps up the output voltage. 7. You must connect the scintillation probe then power up the board connecting the USB cable.
Please do not operate without a scintillation probe connected.
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RGB led:
• Red color means voltage not yet reached or unable to reach. • Green appares when desired voltage is reached. • Blue led blinks only when there is audio data exchange between audio codec and PC. • Some red blinking could be seen even if the green led is steady. This is normal and it means
that the MCU is doing a PWM regualtion. • Various shades of green/red color could be seen. The more reddish the resulting color is,
the more hardly the MCU is working to regulate that particular voltage. Some voltages are more easy to regulate than others.
• At cold start the circuit need some time to reach operative voltage, aproximatively 15/30 seconds.
• You can change voltage selection dip switches during operation or prior powering the board.
• If you select a too high voltage or your photumultiplier base/voltage divider have a too low resistance, an overload of the high voltage power supply happens. The overload protection will drop the voltage down to near zero and auto resets in aproximatively 60 or maximum 90 seconds. If the overload cause is not resolved it will block in low voltage mode. To restart simply disconnect and reconnect the USB cable. During all this situation the red led will be steady red.
High voltage filter
• Removes the 3.9Khz switching frequency noise from the high voltage. It uses a series of two 1Mohm-22nF low pass filters. 54dB attenuation per filtering cell with 108dB attenuation at 3,9Khz. You can make a solder bridge between SJ1-2-3 to select the 100k filtering resistors. This will make the filter capable to output x10 more current to the photomultiplier. The recommended photomultiplier base voltage divider must be around 80-120Mohm. Value less than 50Mohm could be trick to drive, this jumpers are provided for this low value voltage dividers.
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Pulse shaper and amplifier This block extracts pulses from the high voltage line and amplifies it. Pulses are also enlarged and modified to become stretched bipolar pulses. For this reason the maximum suggested counting rate is 500 counts of pulses per second. To optimize the gamma spectrum resolution, set the distance from the probe to the radioactive sample in order to stay around this value of CPS. Lower counting are ok, higher are still ok within certain limits. 600-700 CPS usually give still good results. SJ4-5-6-7 are experimental jumpers already set during production. They enable or disable certain parts of the shaping filter transforming the pulse shape. The standard positions are:
Usualy there is no need to modify this settings. Audio codec and usb This section samples the incoming pulses and outputs it to the PC as audio signal. It’s based on CM108AH audio codec that is capable to sample audio at 48Khz rate with 16bit resolution. You will see it into your computer USB device list as Plug-n-play USB audio device. If your computer don’t find the driver for the CM108AH codec, you can download it here: https://www.cmedia.com.tw/products/USB20_FULL_SPEED/CM108AH How to use?
1. Connect a scintillation probe with a base/voltage divider of recomended 100Mohm resistance. Use only a 75Ohm RG59 cable with BNC male connector of excellent quality. Remember that into the cable will be many hundred volts or more than 1 kilovolt! Use only a known brand of cable/connectors source like Amphenol.
2. Select the desired operation voltage with dip switch selector. 3. Connect via a USB cable the circuit board to the PC. 4. Wait until red led becomes green, it means desired output voltage is reached. 5. Now the pulses could be recorded by your PC and/or analyzed by a MCA software like
Becquerel Monitor or PRA. 6.
Control pannell -> Hardware and sound
The circuit board is seen as USB audio card
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7. You must regulate the input mike volume of this “audio card” to obtain a linear spectrum. This will take some time and is the trikyiest thing to do. A good starting point is input mike audio gain at 20%. Disable the mike “AGC” automatic gain control.
Disable the AGC control
Set the input gain. Start with 10…25%
8. You must regulate the working voltage of the photomultiplier tube to reach the best working
zone of the tube. Better to stay low with voltage and then increase it gradually if needed. 9. Indicatively Hamamatsu R6095 works at 600V @ 25% audio gain. R980 550V @ 35%
gain this is what I’ve tested. 10. For any question feel free to ask me…for the basic settings for your tube maybe.
Becquerel Monitor 2011 software MCA quick start guide Becquerel monitor 2011 is one of the best software MCA avaiable for free. It is simple to use, let’s see how to configure. Connect your USB PMT adapter with a scintillation probe attached and follow this procedure. Download it here: https://madexp.com/wp-content/uploads/2018/05/BecquerelMonitor-1.0.zip It doesn’t need a software installation, it’s a compressed zip folder. Unzip it into your desktop, open it and double clck on the “.exe”. The first step before software adjustment is to set volume level for the spectroscopy device. Set the volume level to avoid overloading or clipping with high and low energy samples. If required, adjust high voltage or pulse amplifier gain. You can be helped to do this by a software audio card oscilloscope like “DAA” that could be downloaded here: https://www.theremino.com/wp-content/uploads/files/DAA_V4.0.zip As first thing to do after opening the software for the first time is to add your scintillation probe.. From the top menù bar select “Edit device configuration”. Press “New”, type the name of the new device and save. You have created your scintillation probe device that will be used by Becqumoni. You can create multiple devices, one for each probe that you have and select wich to use into the Becqmoni software main window.
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Leave inalterated the default settings like “# of Channels” and “Channel Pitch”. Preset time is the integration time required to complete the spectrum data aquisition. You can leave it at 3600 sec and eventually change into the main window if a different time is required. Now press “Device specific” tab.
Device Specific Tab Audio Input Device: select line-in or mic-in input of your audio card Sampling Rate: set 48000 Bit per Sample: set 16 Input Level: set 100% Device Polarity: leave blank. LLD: lowest level to detect, filter out PMT noise on low energy range. Default value 1 ULD: upper level to detect, filter out any noise on the upper energy range, by default not used and set for 100 Shape Threshold: default 60%, used by software to filter out pulses with incorrect shape.
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Now, is time to train the software to recognize valid pulses. Add a radioactive source near the scintillation probe. It could be a thoriated welding rod or gas mantle as example. Set “Sample with” to 32 and “Peak Position” 8. LLD is noise discriminator treshold. Set a bit high like 4 then press “Start”. You must see the complete pulse into the black pulse shape viewer. Now you must wait until a sufficent number of pulses are recognized like 1…3000 pulses. Once the “standar pulse shape sampling” is complete, press “Save”. Now you can “Close”. Each time you modify the device settings, you must reload the device configuration pressing the circular arrow on the right of the “Device configuration” selection menù. To run a new gamma ray spectrum analisys press “File” -> “New”. Select the name of the scintillation probe device that you have created previously into “Device config” tab. Close “ROI configuration” pressing it’s “X”. Now you are ready to press “Start”. If the 3600 seconds of acquisition time is of for you, leave it. For a quick test use 1200. You can pause and restart the data aquisition with “Stop” and “Start” buttons. The longer acquisition time, the more precise will be the spectrum. Repeat initial volume adjustments or spectroscopy hardware calibration until you get all requested energies within detectable range. In some circumstances you may need to change LLD or UDL levels to cut hardware noise. If you change volume level, amplifier gain, high voltage or pulse duration then pulse shape training have to be repeated.
High voltage output test chart All values are in Volts measured with a 110Mohm voltmeter. First value from 500 to 1000V are without voltage boost enabled. Values from 1000 to 1400V are with voltage boost activated.
Voltage selection table 0=Pin up 1=Pin down. HV_BOOST enabled: add 500V max voltage output: 1450V The voltage at the input of the high voltage filter is higher than output. At max output voltage of 1450V the input voltage is 1650V. The air moisture, dielectric leaks and component tollerances are the cause of this voltage loss that is not a fault, but a well know characteristic of the filter design.
