LARGE VOLUME HEMISPHERICAL NUCLEAR RADIATION DETECTOR CZT/500(S)

Transcription

1 LARGE VOLUME HEMISPHERICAL NUCLEAR RADIATION DETECTOR CZT/500(S) Ganibu Dambis 26, P.O.Box 33 Riga, LV-1005, LATVIA Tel. (+371) Fax:(+371) CONTENTS INTRODUCTION... 2 SPECIFICATION... 3 DESIGN FEATURES OF CZT/500(S)... 4 SAFETY AND PRECAUTION... 5 THEORY OF THE HEMISPHERICAL DETECTOR OPERATION... 5 DELIVERY SET PRICE...Error! Bookmark not defined. OTHER CONDITIONS... 9 ILLUSTRATION Fig. 1 - CZT/500. Fig. 2 - Spectrum of Cs 137 measured with CZT/500. Fig. 3 - Design features of CZT/500(S). Fig. 4 - Cross sectional view of a hemispherical detector showing the drift of electrons toward a positive point electrode and the resulting induced charge pulse. The majority of the induced pulse arises from electrons originating in the cross-hatched region of the hemisphere. Fig. 5 - Appearance of a quasi-hemispherical detector. Head Management: Dr. Gostilo Vladimir Dr. Sokolov Alexander Mr. Moshak Vladimir Bank details: NORDEA Bank Finland Plc Latvian branch Bank code: NDEALV2X IBAN account: LV13NDEA Commercial registration number: VAT Registration No. LV Place of registration: Riga, Latvia

2 INTRODUCTION The CZT/500(S) is a nuclear radiation detector based on CADMIUM ZINC TELLURIDE -CdZnTe, a large bandgap semiconductor material having a high atomic number and high density, which makes these detectors one of the most sensitive small sized and room temperature detectors in operation. The CZT/500(S) is a spectrometric detector intended for gamma radiation registration in a range of an energy registered more then 60 kev. Availability of the letter S in the detector's name means Super Grade (detector with high spectroscopy performance) detector CZT/500S. Outward appearance of the CZT/500 is shown in fig.1. Fig. 1 - CZT/500. Typical spectrum of Cs 137 measured with CZT/500 is shown in fig. 2. Fig. 2 - Spectrum of Cs 137 measured with CZT/500.

3 SPECIFICATION Basic Parameter Value detector type CdZnTe detector geometry quasi - hemispherical detector sensitive volume 500 mm 3 Performance (at operation temperature +22 C) Parameter energy resolution (FWHM) at 662 kev line for CZT/500 for CZT/500S peak-to-compton ratio at 662 kev line for CZT/500 for CZT/500S Value < 30 kev < 18 kev > 2.3 > 4.0 Bias voltage requirements Parameter detector high voltage polarity Value positive Dimensions Parameter diameter length (without connector) distance between a top plane of the housing cover and sensitive surface of the detector Connector BNC type Value 23 mm 33 mm 7 mm Typical cable lengths: 0.2 m (available from 0.1 to 30 m) Typical connectors: single 5-pin LEMO or 9-pin D type, BNC and SHV.

4 DESIGN FEATURES OF CZT/500(S) The CZT/500(S) design features are shown in fig. 3. The detectors consist of a CdZnTe detector, watertight housing and connector. Fig. 3 - Design features of CZT/500(S).

5 SAFETY AND PRECAUTION Equipment Precautions: The CZT/500(S) must be connected to the POSITIVE high voltage supply. The detector housing is fragile and should not be strongly squeezed. When possible, use radiation shield and (or) collimator or maintain the detectors a distance from strong neutron-gamma sources for prevention of detector and connector s insulator radiating damages. Decontamination or cleaning of detectors can is carried out with water or other non-corrosive liquids. Radiation Dose Caution: The detector should be routinely checked for contamination and decontaminated if necessary after use to avoid possibility of radioactive contamination. THEORY OF THE HEMISPHERICAL DETECTOR OPERATION The resolution of the wide band semiconductor detectors, such as CT or CZT detectors optimized by choosing a small hemispherical crystal geometry with a positive contact at the center of the flat surface and the outer spherical surface grounded (see fig. 4). The electric field is essentially radial and therefore much stronger near the positive contact. Assume first that only electron charge collection occurs with no appreciable electron carrier trapping. The pulse that is registered from a photoelectric event with a constant number of electrons produced would arise mainly from the induced charge from the electron carriers traversing the high field region. Even so, gamma photoelectric interactions in the lower field region (cross hatched) constitute the majority of the peak area (cross hatched) since this is where the majority of the detector volume resides. The electron carriers from throughout the detector volume drift toward the positive electrode reaching their highest velocity near the positive electrode. The electron collection time is less then 0.5 ms. Fig. 4 - Cross sectional view of a hemispherical detector showing the drift of electrons toward a positive point electrode and the resulting induced charge pulse. The majority of the induced pulse arises from electrons originating in the cross-hatched region of the hemisphere.

6 The pulse height from a gamma interaction does not appreciably depend on holes collection due both to the hemispherical geometry and the short pulse shaping time. This is fortunate since hole drift velocities are an order of magnitude smaller causing hole collection time to be typically 5 ms and hole trapping to become important. The hole pulse height contribution tends to be very small since few holes traverse the high field region drifting rather toward the negative hemispherical surface electrode, thus a far smaller pulse is induced and the pulse shaping time of about 1 ms clips this pulse contribution well before it reaches its full height. The 1 ms shaping time does no clipping to the electron collection signal. The CZT or CT detector resolution is limited primarily by trapping of holes due to impurities and inhomogeneities in the crystal. Manufacturing of detectors with ideal hemispherical geometry is labor-consuming process. Therefore in detection units are used quasi-hemispherical detectors. The appearance of such detectors is shown in fig. 5. Researches have shown that the replacement of ideal hemispherical geometry of the detector on quasi-hemispherical a little bit worsens the spectrometer performance. contacts Fig. 5 - Appearance of a quasi-hemispherical detector.

7 Digital Miniature Multi-channel Spectrometric Device MCA-527 The MCA527 is a battery powered high performance 8K Multi-Channel Analyzer/Multi- Channel Scaler module with the performance of a laboratory grade MCA. High voltage supply for detector and preamplifier power supply are integrated as well as an internal coarse amplifier and digital filtering and analysis. Together with a detector it forms a small-size gamma spectroscopy system, which is well suited to the demands of field measurements for international safeguards, environmental monitoring, nuclear waste treatment facilities, radioactive transport control and similar applications Hardware specification Amplifier: Coarse amplifier prefilter with amplifications in steps of , corresponding to a full scale ADC; Input signal positive or negative; linearity better than 0.1%. ADC: 14bit, 10 MSps; Integral Nonlinearity <0.05%; temperature stability TK 50. Digital signal processing: double differential trigger filter, or single differential low energy low count rate trigger filter; Pile-up-suppression, pulse pair resolution ~400ns, depending on trigger filter; Automated and manual adjustment of trigger threshold; Channel splitting 128, 256, 512, 1k, 2k, 4k or 8k; Differential nonlinearity <1% for 4k channels and 2µs peaking time; Automated PZC adjustment, detector decay time constants from 5µs to 1ms can be compensated. High voltage: Detector HV up to ±3.6kV, polarity depends on module inserted. Power supply: Li-Ion batteries, operation time 10-25h, depending on detector connected (tbd). Computer Interface: USB, RS-232 (38.4, 115.2, kbd and 3MBd), Ethernet. Mechanical: Housing 164 mm x 111 mm x 45 mm without connectors; weight 820g.

8 Environmental: operational: at least 0-50 C, eventual larger range; humidity up to 90%, non condensing, IP42. SpectraLineGP software package Spectra processing of the SpectraLine software package includes calibration, peaks parameters determination, nuclides identification, activities calculation and using the true-coincident factors for the gamma-emission intensity correction. Non-parametric model for pattern of the full energy peak provides a correct model for a line in any energy range. SpectraLine software package enables to use external programs as an additional instrument for user methodics realization of the specific spectrometric problems. Software for spectrometer emulation Features: to execute spectra acquisition for the set time, to select interesting spectrum sites and examine them on a separate plane, increasing or reducing scale on horizontal and vertical; to execute power calibration of spectra on two known energies; to determine centroids and area of peaks with background deduction and without background deduction; to carry out an automatic serial spectra acquisition (not less than 100) with automatic record on a disk; to mark and select with color peaks or interesting spectrum areas; to move a marker on a spectrum or interesting areas; to make an estimation of energy resolution at one second and one tenth height of full absorption peak; to print out spectrum window a on the matrix or laser printer; to transfer the spectrum to a software for identification of radio-nuclides and calculation of their activity. Gamma Analysis Software (SPECTRALINE Software) Features: automatic peaks search with the required sensitivity level; the energy, FWHM and peak pattern calibrations; peaks parameters determination - position, FWHM, area; calculation results can be saved to a text file; efficiency curves can be obtain with the efficiency calibration; activity calculation by various methods;

9 use the true-coincident factors to correct the gamma-emission intensity; the measured spectra and processing results saving to database for the analysis of the specified criterions convergence of the repeated measurements; simultaneous processing of any spectra number; the peak pattern calibration with several spectrum peaks in different energy ranges; numeric and visual control of the calibrations results; any measurement tracts number; simultaneous spectrum acquisition and visualization of all measurement tracts attached to the computer; adjustment of the ADC parameters; independent control of all channels - start, stop, etc.; support of the spectrometry devices of different developers. Alexander Sokolov Technical director