2006 DOE NEER Final Report - Kansas State University Mechanical ...

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samples were rinsed with DI water and isopropyl alcohol before chemical treatment. The rinse was followed by a chemical etching step with a 2% bromine/methanol solution for 2- minute duration. The samples were then rinsed once more with isopropanol. Gold was deposited on the ends of the crystal to form the ohmic contacts through an electroless deposition technique using gold chloride (AuCl 3 ). The AuCl 3 solution was deposited for eight minutes on each contact. When gold chloride is applied to the CdZnTe surface, a chemical reaction occurs, in which cadmium is removed from the surface and replaced by gold. Surface passivation was performed as the final treatment to some of the devices. An ammonium fluoride (NH 4 F)/DI water/hydrogen peroxide (H 2 O 2 ) solution of 2.68 g/17 ml/8ml was applied to the crystal surfaces for 10 minutes. Passivation provides an oxide layer that replaces tellurium rich surfaces left behind by the bromine/methanol etching step. This resistive oxide reduces side-surface leakage current substantially. Finally, the passivated crystal was wrapped with thin Teflon tape, which acted as an insulating boundary. The Teflon was extended beyond the anode about 2 mm in order to avoid discharge between the collar and the anode. A thin copper shim was cut to size and used as the Frisch collar . The Frisch collar extended the entire length of the device, and was connected to the device cathode. In this manner, the Frisch collar was held at the cathode potential (or ground). C. Results The results of this report is presented in three sessions as the spectroscopy results, angular dependency results of collimated device and charge collection characterization results. The details of the experimental setup for each study have been reported elsewhere [15-17]; however, the schematic setup of each study are presented in figures 1 through 3. C2. Spectroscopy results Pulse height spectra were collected from gamma rays sources of 241 Am, 57 Co, 133 Ba, 198 Au, 137 Cs and 235 U. The 241 Am, 57 Co, 133 Ba and 137 Cs samples were standard commercially available calibration sources. The 198 Au was prepared through an (n,γ) reaction by irradiating a 13 mg sample of gold foil in the Kansas State University TRIGA Mk II nuclear reactor core 10
for 5 minutes at a fast neutron flux of 3.5x10 12 cm -2 s -1 and thermal flux of 4.3x10 12 cm -2 s -1 . The 235 U source was in the form of a 93% enriched 235 U-nitride solution. The electronic settings were consistent as previously explained for all measurements and energy resolutions reported for gamma ray photopeaks are without any electronic corrections. An energy resolution of 9.10 % (5.41 keV) FWHM for the 59.5 keV spectral line of 241 Am was obtained. The evaluated energy resolution is shown on Fig. 4 and does not take into consideration any contributions from electronic noise. The low noise floor and the high resolution of the device allows for the discernment of Cd and Te X-ray escape peaks, as labeled near 35 keV. A 57 Co spectrum is shown in Fig. 5. The energy resolution at the 122 keV photopeak is 4.91% (5.99 keV) FWHM and the 136 keV photopeak is clearly observed as a small peak on the right side of the 122 keV photopeak. The Cd and Te X-ray escape peaks are also visible. An electronic pulser peak is shown in the spectrum for noise comparisons, and is located near 191 keV on the energy scale. Figs. 6 through 8 display the energy spectra for 133 Ba, 198 Au, and 137 Cs, respectively. An energy resolution of 7.55 % (6.04 keV) FWHM was achieved for the 80 keV photopeak of 133 Ba as shown in Fig. 6a. The 356 keV photopeak of 133 Ba exhibited an energy resolution of 2.33% (8.29 keV) FWHM (see Fig. 6b). The full energy peaks corresponding to the 276 keV, 302 keV and 382 keV emission energies of 133 Ba are also clearly present in Fig. 6b. The 20 keV low noise cut off makes the device capable of detecting the cesium (Cs) X-rays seen in Fig. 6a. The 198 Au spectrum is illustrated in Fig. 7. The 412 keV full energy peak resolution is 2.26% (9.31 keV) FWHM. Again, the Cd and Te X-ray escapes are observable. The 662 keV photopeak of 137 Cs is observed with 1.45% (9.6 keV) FWHM energy resolution. The Compton continuum and backscatter peak are clearly seen in Fig. 8. Due to low noise, barium X-rays are also easily identified. Fig. 9 shows the energy spectrum of 235 U obtained from a low-activity 93% enriched 235 U-nitride solution. The distance between the detector and the source was approximately 5 cm and the exposure time was 20 hours. All expected energy emissions, along with their branching ratios, are illustrated in Fig. 9. The clearly distinguishable spectral lines of 235 U (Fig. 9) confirms that the insulated Frisch collar design incorporating bar-shaped CdZnTe material is fully capable of radioactive material identification. 11
Page 1 and 2: Semiconductor Radiation Detectors w
Page 3 and 4: their own material for this work. T
Page 5 and 6: cont. 2. A. Kargar, A.M. Jones, W.J
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Page 9: trapping [3-9,11-14]. A relatively
Page 13 and 14: collimators were not taken into con
Page 15 and 16: F. Figures Fig. 1. The Experiment a
Page 17 and 18: Fig. 4 Room temperature spectra of
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Page 25 and 26: (a) (b) Fig. 19. The 2x2 module of

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