CZT D ETECTOR T ESTING USING N UCLEAR R ADIATION

In this study, the CZT detectors were developed for detection of gamma radiation from nuclear materials. Gamma radiation has frequencies of above 1019 Hz and energies typically above 10 keV. In order to detect gamma radiation, first detector materials absorbs the gamma radiation to produce fast moving electrons within the detector by one of three methods: photoelectric absorption, Compton scattering, and electron-positron pair production [3]. Thus the photon energy of the gamma-ray is converted into electron energy. By applying an external voltage bias, the fast moving electrons can be collected at an electrode (anode), inducing a charge on the contact electrodes of the detector, which is then read out by the front-end detection electronics to provide pulse height spectra (PHS) for the incident radiation. Front-end electronics (Figure 2.23) consist of preamplifiers which converts charge signal to a voltage signal, shaping amplifier which filters noise, and multi-channel analyzers (MCA) which converts analog signals into digital information as pulse height spectrum.

The analog radiation detection experiments are conducted using a Canberra 3106D high voltage supply which biases the CZT radiation detector through an SHV bulkhead. The CZT detector is housed in an aluminum RFI/EMI shielded test box. Inside the box, the detectors are placed either in a PCB holding mount, or one electrode is placed on a gold foil test pad and the other electrode is connected to a pogo-pin contact. Underneath the detector, a 241Am or 137Cs nuclear source is placed to irradiate the detector. 241Am provides low-energy gamma-rays at 59.6 keV or alpha particles at 5.486 MeV, while 137Cs is used for high-energy gamma-rays at 662 keV. Figure 3.27 (a) shows the basic schematic configuration of the electrical connections to the CZT detector. The

shielded aluminum test box (Figure 3.27 (b)) is connected to an Amptek A250CF preamplifier through a BNC, which is then connected to an Ortec 671 spectroscopic shaping amplifier. The shaping amplifier is then connected to an oscilloscope and a Canberra Multiport IIe multi-channel analyzer (Figure 3.27 (c)). Data from the multi- channel analyzer is sent to the Genie 2000 PC software, which generates the pulse height spectrum. The radiation detection setup in our laboratory at USC is shown in Figure 3.27.

Figure 3.27. (a) Basic connection diagram for a CZT nuclear detector inside of the

shielded test box, (b) picture of the shielded aluminum testing box with CZT detector, and (c) picture of the radiation detection system at USC.

(b)

(c) (a)

Figure 3.28. Schematic diagram of a digital nuclear detection measurement system at USC.

Once pulse height spectrum was generated, the full width at half maxima (FWHM) of the gamma-ray energy peak was calculated through Gaussian peak fitting using the Origin plotting software. The energy resolution of the detector is calculated by the following equation:

% 𝐸𝑛𝑒𝑟𝑔𝑦𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛=_{𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡}𝐹𝑊𝐻𝑀_{𝐸𝑛𝑒𝑟𝑔𝑦} (𝑘𝑒𝑉 ₍)_{𝑘𝑒𝑉)}∗100%

3.17 where the incident energy is the centroid of the energy peak observed in the pulse height spectrum. Lower values of energy resolution and FWHM indicate better detector performance.

The planar CZT MSM detector and CZT detector with guard ring were first tested using the 241Am (59.6 keV) source to test its response to low-energy gamma-rays. Figure 3.29 and Figure 3.30 show the resulting pulse height spectrum with 241Am for planar detector and detector with guard ring, respectively. After performing Gaussian peak fitting, the FWHM of the gamma photopeak at ~59.6 keV was calculated to be ~6.2% and 5.8% respectively. Both these detectors clearly detect 59.6 keV energy,

however the detector with guard ring showed a much sharper peak. The peaks are more resolved (low noise) due to lower leakage current observed for this detector.

Figure 3.29. Pulse height spectrum (PHS) of the CZT Schottky diode detector with a resolution of 6.2% at 59.6 keV using a 241Am radiation source.

Figure 3.30. Pulse height spectrum (PHS) of the CZT Schottky diode detector with guard ring using a 241Am radiation source. The peaks are more

resolved due to lower leakage current (low noise). A resolution of 5.8% at 59.6 keV is observed. γ = 59.6 keV FWHM = 6.2% Detector: CZT Planar Source: 241-Am Shaping time: 1ms Bias voltage:-600 V Acquisition time: 120 s

Detector: CZT Planar with Guard Ring Source: 241-Am Shaping time: 8s Bias voltage:-100 V Acquisition time: 120 s γ = 59.6 keV FWHM = 5.8% 1µs

Finally, CZT planar detector with guard ring was tested using 137Cs (662 keV) source to test its response to high-energy gamma-rays. Figure 3.31 shows the resulting pulse height spectrum with 137Cs for planar detector with guard ring. The data shows a sharp 662 keV energy peak. After performing Gaussian peak fitting, the FWHM of the gamma photopeak at 662 keV was calculated to be 2.6%.

Figure 3.31. Pulse height spectrum obtained for CZT planar detector with guard ring using 137Cs gamma radiation source.

Detector: CZT Planar with Guard Ring

Source: 137-Cs

Shaping time: 1s

Bias voltage:-1800 V

3.8

C

CZT crystal was grown at a stoichiometry of Cd0.9Zn0.1Te from zone refined ultra-

pure precursor materials with 50% excess Te using modified multi-pass vertical furnace. The bandgap of the crystals was found to be 1.56 eV, which is in the correct range for detector-grade CZT. Defect analysis was performed on the grown CZT crystals using TSC and EBIC analysis. TSC experiments revealed deep-level defects in the crystal which contribute to hole trapping. EBIC results showed that clusters of dislocations and point defects within the bulk of the CZT crystals due to Te segregation. These results give insight on the type and severity of defects present within the solution-growth CZT crystals, which may assist in reducing defects present in future crystal growths.

The electrical resistivity was estimated to be 6 × 1010Ω-cm, which is high enough to fabricate a functional CZT radiation detector. The CZT detectors showed very low leakage current at a high bias (<5 nA at –1000V) due to their high resistivity, which are beneficial for high resolution detectors. The drift mobility and mobility-lifetime product of electrons were estimated to be 1186 cm2/Vs and 5.9 × 10-3 cm2/V, respectively. These data provides an insight on the potential performance of the CZT nuclear detectors, and ensures that only the best samples are chosen to be fabricated into detectors.

Finally, CZT detectors were tested with gamma-ray irradiation. An energy resolution of 6.2% was obtained for CZT planar detector when irradiated with 59.6 keV low-energy gamma radiations (241Am). The peaks were sharper and better resolution of 5.8% was observed for the CZT detector with guard ring with 59.6 keV radiation. An energy radiation of 2.6 % was observed for detector with guard-ring structure irradiated with high energy 662 keV gamma radiations using 137Cs source.