X-Ray Spectroscopy with PIN diodes F. J. Ramírez-Jiménez Instituto Nacional de Investigaciones Nucleares Carretera México-Toluca S/N, La Marquesa, Ocoyoacac, 57150, MEXICO e-mail: [email protected]Abstract. A PIN diode and a low noise preamplifier are included in a nuclear spectroscopy chain for X-ray measurements. This is a laboratory session designed to review the main concepts needed to set up the detector-preamplifier array and to make measurements of X-ray energy spectra with a room temperature PIN diode. The results obtained are compared with those obtained from radioactive sources with a high resolution cooled Si-Li detector. Keywords: PIN diodes, X-rays, spectroscopy. PACS: 29.40.-n, 29.30.Kv INTRODUCTION An experiment is proposed as a laboratory session, in which a PIN diode and a charge sensitive preamplifier specially designed to match the PIN diode characteristics are included in a nuclear spectroscopy chain to measure the X-ray energy spectra from radioactive sources, the PIN diode is operated at room temperature. The main concepts needed to set up the detector-preamplifier array are reviewed. The energy resolution of the system is measured and the results obtained are compared with those obtained with a high resolution cooled Silicon Lithium-Drifted, Si- Li, detector. X–ray spectroscopy is useful to identify radioactive sources by the energy of the peaks encountered in the spectrum; also it is utilized to determine the elementary composition of samples by X-ray fluorescence, in this
Transcript
X-Ray Spectroscopy with PIN diodes
F. J. Ramírez-Jiménez
Instituto Nacional de Investigaciones NuclearesCarretera México-Toluca S/N, La Marquesa, Ocoyoacac, 57150, MEXICO
Abstract. A PIN diode and a low noise preamplifier are included in a nuclear spectroscopy chain for X-ray measurements. This is a laboratory session designed to review the main concepts needed to set up the detector-preamplifier array and to make measurements of X-ray energy spectra with a room temperature PIN diode. The results obtained are compared with those obtained from radioactive sources with a high resolution cooled Si-Li detector.
An experiment is proposed as a laboratory session, in which a PIN diode and a charge sensitive preamplifier specially designed to match the PIN diode characteristics are included in a nuclear spectroscopy chain to measure the X-ray energy spectra from radioactive sources, the PIN diode is operated at room temperature. The main concepts needed to set up the detector-preamplifier array are reviewed. The energy resolution of the system is measured and the results obtained are compared with those obtained with a high resolution cooled Silicon Lithium-Drifted, Si-Li, detector.
X–ray spectroscopy is useful to identify radioactive sources by the energy of the peaks encountered in the spectrum; also it is utilized to determine the elementary composition of samples by X-ray fluorescence, in this technique an excitation source is employed. The excitation source could be: an X-or gamma-ray radioactive source or an X-ray tube.
The main characteristics of X-rays are the energy and the intensity. The energy is in the order of some keV’s and depends of the energy of the accelerated electrons in case of generation of X-rays through a collision process. In the case of X-ray fluorescence or nuclear decay, the energy depends of the chemical element. Radioisotope X-ray sources generate characteristic X-rays with well defined energy peaks. These peaks are very useful as absolute energy reference points to make the energy calibration of a spectroscopy system. The number of counts registered in the spectrum depends on the activity of the source.
In the nuclear decay process of an atom, electron capture and internal conversion, lead to the generation of X-rays which are characteristic of the final element of the decay. Radioisotopes that follow this process are sources of characteristic X-rays.
A basic energy spectroscopy system is shown in the Figure 1. The signal of charge generated in the detector by the radiation is conditioned in the preamplifier, the amplifier gives a further amplification and sets the optimal bandwidth of the system in order to get the best signal to noise ratio, the output from the amplifier is a series of analog pulses with a height directly proportional to the energy of the detected radiation, these pulses are analyzed considering its pulse height, classified an the result of this classification is shown in the screen of the multichannel analyzer system.
FIGURE 1. Basic Energy Spectroscopy System.
The energy spectrum of an Am-241 radioactive source is shown in the Figure 2, a cooled Si-Li detector was employed to realize the measurement. The characteristic peaks of the radioactive element are clearly defined.
FIGURE 2. X-ray energy spectrum of an Am-241 source, obtained with a Si-Li detector.
The total noise is one of the more important characteristics of the spectroscopy system and is reflected on the spectrum in the width of the peaks, the energy resolution of the system is defined [1] as the Full Width at Half Maximum of a peak, FWHM, and can be measured directly from the spectra, see Figure 3. The resolution for X-ray detectors is measured in the peak, Ho, of 5.89 keV of a Fe-55 radiation source.
FIGURE 3. Definition of the energy resolution (FWHM) in a spectroscopy system.
The total noise is the resultant of two uncorrelated components: the electronic noise associated mainly with the preamplifier and the electrical characteristics of the detector, and the statistical component due to the detection process inside the detector.
(1) (2)
where: F=0.12 is the Fano factor for Silicon, E is the energy of the incident photons in eV and is the energy needed to create an electron-hole pair; = 3.6 eV for Silicon detectors [2].
PIN DIODE DETECTORS
Si-Li detectors are normally used to get the best results in X-ray analysis due to its good energy resolution, typically 180 eV, and good efficiency in the energy range from 1 kev to 100 keV, nevertheless there are other kind of detectors commercially available and suitable for X-ray detection, such as: semiconductor drift detectors [3][4] and special PIN diodes [5] [6], these detectors have some nice advantages over the Si-Li detectors mainly with respect to the size.
We demonstrate the detection capability of an OPF420 PIN photo diode [7], originally intended for optical fiber applications, in the proposed experiment. The basic structure of a PIN diode is seen in Figure 4(a), the three regions, n+, Intrinsic and p+, can be distinguished clearly. Figure 4(b) shows the real active area of the PIN diode. The mechanical characteristics of this device are: 300 m of active thickness, 1 mm2 active area, and a glass window of 1 mm thick.
(a) (b)
FIGURE 4. a) Basic structure of a PIN diode, b) Picture of an OPF420 PIN diode as seen through a microscope.
Semiconductor diode detectors need to be operated with enough reverse voltage in order to reach the maximum depletion zone and therefore to get the best detection efficiency because in the depletion zone occurs the generation of carriers by effect of radiation in the best conditions. The Figure 5 shows the evolution of the radiation effect in the detector, an electric charge Qi is induced first by the movement of the generated electrons and then by the movement of holes, both of them generate a pulse of current.
FIGURE 5. Signals generated inside the detector. Electric charge induced by the radiation in the PIN diode and the corresponding generated current.
PIN diodes can be represented by an electrical model as shown in Figure 6, in this model the main components are the equivalent charge source Qi, the leakage current Io
and the diode capacitance Cj. The PIN detector has been characterized as X-ray detector for this application [8];
the experimentally obtained electrical characteristics are shown in Figure 7. The graph of variation of the detector capacitance Cj and leakage current Io, as a function of reverse voltage is used to determine the optimal operating voltage of the PIN diode. The selected point is where the reverse current does not increase abruptly and the diode capacitance gets a minimum, which implies that full depletion is reached. In this case, the operating point is 65 V, where the leakage current is less than 700 pA and the capacitance is less than 2 pF. From these values, we concluded that the PIN diode can be used at room temperature with good performance for X-ray spectroscopy
because its leakage current is small, but it requires a low input capacitance and low noise preamplifier as read-out circuit.
FIGURE 6. Electrical model of a PIN diode in reverse bias mode.
FIGURE 7. Dependence of diode capacitance and leakage current with the reverse voltage for a PIN diode.
The detection efficiency of radiation detectors needs to be specified. The intrinsic efficiency is defined as the ratio between the detected and the total number of events reaching the detector; it was experimentally obtained for the PIN diode by using the standard radioactive sources of the Table 1, with its characteristic energies [1].
Table 1. Characteristics of radioactive sources used for calibration.
Results of these measurements are shown in Figure 8, where it can be seen that the range of operation with good efficiency is from a few keV, limited by the window, up to 60 keV when the relative efficiency is still around 2 %. To use the PIN diode for lower energy X-ray measurements, the original glass window can be taken out or replaced by a 25 m beryllium foil.
FIGURE 8. Intrinsic efficiency of a PIN diode with an effective thickness of 300 m.
Principle of Operation
Measurement of the radiation is based in the production of electron-hole pairs in the interaction with the detector material and the further collection of charges (see Figure 5). The number of electron-hole pairs generated [2], is related to the incident energy
, by:
(3)
Then the total charge , generated in the detector by the interaction is: (4)
is the electric charge of the electron. Substituting eq. (3) in eq. (4) we have:
(5)
An important conclusion from this equation is that the generated charge in the detector is directly proportional to the energy of the radiation. Due to the very small signal generated in the detector for X-rays, the noise of the measuring system has to be considered with great care.
CHARGE SENSITIVE PREAMPLIFIERS
The interface between detector and preamplifier defines the low noise of the spectroscopy system, therefore for these applications, the best noise performance of the associated preamplifier is required, generally the preamplifiers have to be done with discrete components selected for low noise. The matching between the detector and preamplifier also defines the noise characteristics of the system, an optimal
matching is desired for low noise performance. Generally, a field effect transistor, FET, is used as the input device.
Special low noise preamplifiers have been developed to fulfill this requirement like the charge sensitive preamplifier in which, the radiation is measured as an individual event, a charge is generated due to the interaction of radiation with the detector and the preamplifier converts the input charge to voltage at the output. The classical charge sensitive preamplifier in one with a feedback resistor, Figure 9, in this case, the feedback capacitor Cf is charged by the injected signal Qi from the detector and discharged immediately through the feedback resistor Rf. The feedback resistor has the disadvantage that it is an additional and undesired source of noise.
FIGURE 9. Charge sensitive preamplifier with resistive feedback.
The charge sensitive preamplifier with optical feedback is another possibility that do not include an additional source of noise by a feedback resistor, Figure 10. The feedback capacitor Cf is discharged through the input FET, when an optically coupled LED sends a reset light pulse to the FET every time the output voltage, Vo, reaches a defined level.
FIGURE 10. Charge sensitive preamplifier with optical feedback reset.
Forward Biased FET Charge Amplifier
We selected a novel preamplifier configuration called forward biased FET charge amplifier, FBFA [9], for the present experiment, in order to get the minimum noise with PIN diodes because this configuration has proven to give a good noise performance when used with silicon diode detectors. The FBFA configuration without feedback resistor but well defined operating point and continuously discharging feedback capacitor Cf is shown in the Figure 11. Cf is discharged through the input field effect transistor biased with the gate in forward mode.
FIGURE 11. Basic idea of the preamplifier with input FET in the condition of forward biased gate.
The output voltage Vo of the preamplifier is:
(6)
where: Qi is the charge generated in the PIN diode by effect of the radiation; A1, B1, s
and 1 are parameters depending on the values of the circuit components. The value at the peak of the signal, see Figure 12, is:
(7)
FIGURE 12. Charge generated in the detector and the corresponding output signal from the preamplifier.
In the FBFA, the gate of the front-end JFET is kept at a constant voltage by slightly forward biasing the gate-source junction, by the effect of the flow of the reverse current of the detector (see Figure 11). The reverse current of the detector plus the current from the gate junction of the JFET will flow through the source to ground. As the charge signal from the detector accumulates in the feedback capacitor , the FBFA will find an equilibrium, through the effect of the AC negative feedback, returning to the quiescent condition.
EXPERIMENT
Equipment Required
- NIM (Nuclear Instrumentation Modules) standard bin- Digital Oscilloscope - Electrometer- Curve tracer for semiconductor devices- NIM nuclear pulse generator- NIM Spectroscopy Amplifier- PIN diode detector with special low noise preamplifier- Multichannel Analyzer (MCA) inside a personal computer- Calibration sources (Cd-109, Am-241, Fe-55)
Experimental procedure
1.- Measure the V-I characteristics of the PIN diode in the curve tracer. A graph similar to the on shown in the Figure 13 should be obtained in the forward bias condition.
FIGURE 13. V-I curve for a Si PIN diode obtained in a curve tracer.
2.- Measure the V-I characteristics of the input FET of the preamplifier in the curve tracer. A graph similar to the one shown in the Figure 14 should be obtained. Register the values of current and voltage of quiescent points shown to calculate the
transconductance gm , of the device. Notice the special situation in the forward bias condition at the gate.
FIGURE 14. V-I characteristics of a FET, showing the forward biased condition of the gate.
(8)
3.- The circuit diagram of the preamplifier to be used is shown in Figure 15, it has the following characteristics: Charge sensitive with a feedback capacitance of 0.045 pF and conversion gain of 22 mV/fC, the equivalent noise referred to the input is 20 electrons (rms).
FIGURE 15. Detailed circuit diagram of the FBFA employed in the experiment.
The preamplifier is assembled in a printed circuit board as seen in Figure 16.
FIGURE 16. Printed circuit board of the charge sensitive preamplifier.
In the FBFA, the gate of the front-end JFET is kept at a constant voltage by slightly forward biasing the gate-source junction, see Figure 17. Bias the preamplifier and measure the voltage in the gate with an electrometer, the value should be around 0.24 V.
0 50 100 150 200 250 300
10
100
1000
Gat
e cu
rren
t ( p
A )
Forward Voltage Vgs( mV )
FIGURE 17. Forward bias condition in the gate of the input FET of the preamplifier.
4.- Measure the conversion gain GV of the preamplifier and compare the result with the theoretical value, according to equation (7):
(9)
the input charge is injected with a pulse generator, see Figure 11, trough the test capacitor Ct . Then, the input charge Qi is:
(10)
5.- Assemble the spectroscopy system as indicated in Figure 1. Standard NIM modules are used.
6.- Adjust the controls of the amplifier to have a time constant between 1 and 2 s, a gain enough to have the 59.54 keV peak of Am-241 in channel 1700 of the MCA used with 2048 channels.
7.- Measure the signals in the outputs of the preamplifier and amplifier. Adjust the pole-zero condition by seeing the signal in the output of the amplifier.
8.- Get a spectrum at least with two known energies (one or two calibration sources). Accumulate at least 10 000 counts in the region of interest (ROI) under the peaks. Register the data in Table 2:
Table 2. Data for energy calibration.Source Energy (keV) Channel Number
With this data calculate the calibration parameters according to the straight line equation:
(11)
9.- Calibrate the system by using the software of the MCA, compare the results.
10.- Obtain the resolution of the system with a Fe-55 source.
11.- Get a spectrum for an Am-241 source. Save the spectrum in ASCII format. The obtained spectrum should be similar to the one shown in the Figure 18(a).
FIGURE 18. a) Raw spectrum obtained with the PIN diode. b) Corrected spectrum considering the intrinsic efficiency of the PIN diode.
Considering the intrinsic efficiency, shown in the Figure 8, the spectrum can be corrected to get the real energy distribution of the radiation as shown in the Figure 18(b).
12.- The spectrum obtained for the Am-241 source with the PIN diode can be compared with the one obtained with a Si-Li detector, as shown in Figure 19. In this figure a good correspondence of the peaks position in both detectors is observed, which guaranties the good linearity of the PIN diode experimental results.
FIGURE 19. Comparison of the spectra obtained with the PIN diode and a Si-Li cooled detector, energy spectra for an Am-241 source.
REFERENCES
1. Knoll G. F., “Radiation Detection and Measurement”, John Willey and S. New York, 2000.2. G. Bertolini, A. Coche, “Semiconductor Detectors”, Edited by John Wiley and Sons, Amsterdam,
Netherlands, (1968).3. Gatti E., Rehak P. “Semiconductor Drift chamber- an application of a novel charge transport scheme”
Nucl. Inst. and Meth. in Phys. Res. 225 (1984) 608-614.4. KETEK, available in the web site: http://www.ketek.com/5. Hamamatsu, available on web site: http://usa.hamamatsu.com/cmp-detectors/xray.htm6. AMPTEK Inc, available in the web site: http://www.amptek .com/ 7. Optek Technology, Inc. Optek’s Product Catalog , Carrollton, TX, USA, 1997. 8. F. J. Ramírez-Jiménez, R. López-Callejas, A. Cerdeira-Altuzarra, J. S. Benítez-Read, M. Estrada-
Cueto, J. O. Pacheco-Sotelo, “PIN diode-preamplifier set for the measurement of low energy and X-rays”, Nucl. Inst. and Meth. A497 (2003) 557-583.
9. G. Bertuccio, P. Rehak and D. Xi, “A novel charge sensitive preamplifier without the feedback resistor”, Nucl. Inst. and Meth., A326 (1993) 71-76.
