Coaxial High Purity Germanium Detector Research Articles

Delbrück scattering of 1.115 MeV photons

The contribution of Delbrück scattering to the elastic scattering of photons has been investigated for photon energy of 1.115 MeV. Differential cross-sections for the elastic scattering of 1.115 MeV photons, using gamma-ray isotope 65Zn, have been measured from high- Z target atoms ( Z = 74 and 82) at angles ranging from 30° to 135°, using a high-purity co-axial germanium detector and computer-based data acquisition hardware and data analysis software. S-matrix theoretical calculations of Rayleigh scattering cross-sections were performed at 1.115 MeV and the experimental results were compared with these theoretical computations, which include Delbrück and nuclear Thomson amplitudes. The present experimental data at 1.115 MeV indicate that Delbrück amplitudes calculated with lowest-order Born approximation are sufficient, as at 1.33 MeV, when combined with S-matrix-Rayleigh scattering amplitudes.

Gamma-ray imaging with a coaxial HPGe detector

We report on the first experimental demonstration of Compton imaging of gamma-rays with a single coaxial high-purity germanium (HPGe) detector. This imaging capability is realized by two-dimensional segmentation of the outside contact in combination with digital pulse-shape analysis, which enables to image gamma-rays in 4π without employing a collimator. We are able to demonstrate the ability to image the 662 keV gamma-ray from a 137Cs source with preliminary event selection, with an angular resolution of 5° and a relative efficiency of 0.3%. This efficiency expresses the fraction of gamma-rays that can be imaged, out of the total gamma-ray flux which is emitted into the solid angle of the detector. In addition to the 4π imaging capability, such a system is characterized by its excellent energy resolution and can be implemented in any size possible for Ge detectors to achieve high efficiency.

Delbrück contribution in the elastic scattering of1.115−MeVphotons

Differential cross sections for the elastic scattering of 1.115-MeV photons from tungsten (Z=74) and lead (Z=82) have been measured at angles ranging from 30 deg. to 135 deg., using a high purity coaxial germanium detector. The experimental results are compared with S-matrix theoretical calculations of Rayleigh scattering cross sections, which also include contributions arising from the nuclear Thomson amplitudes and the Delbrueck amplitude in lowest order Born approximation. The present experimental data at 1.115 MeV indicates that Delbrueck amplitudes calculated with lowest-order Born approximation, when combined with S-matrix Rayleigh scattering amplitudes, are sufficient, as has previously been observed at 1.332 MeV for a number of high-Z elements, and at 1.121 MeV and 1.173 MeV for Z=92. This result for Z=74 and Z=82 at 1.115 MeV provides further confirmation that the Delbrueck amplitudes calculated with lowest-order Born approximation are sufficient for energies at and below 1.332 MeV, in contrast to the situation at 2.754 MeV where Coulomb corrections to the Delbrueck amplitudes are significant for high-Z elements.

An empirical function for the full energy peak efficiency for a P-type Co-axial high purity germanium detector

An empirical analytical function has been obtained for the full energy peak efficiency of a co-axial P-type High Purity Germanium Detector for gamma-ray energies in the range of 80 - 1333 keV and source-to-detector distance in the range of 1.0 - 4.0 cm. Comparison of the calculated efficiencies with the corresponding experimental value agrees very well within standard deviations of 1.8 - 3.7 %. JOURNAL OF THE GHANA SCIENCE ASSOCIATION Volume 2 No. 1 (2000) pp. 64-70

Radioactivity in the community water supplies of Ife-Central and Ife-East local government areas of Osun State, Nigeria

The average concentrations of radionuclides in the various types of community water supplies of the Ife-Central and Ife-East Local Government areas with a population of 200 000 people were estimated from the measurements of mean specific activity using a well-calibrated Canberra vertical coaxial high-purity germanium detector system. Water samples were collected from dam, streams, boreholes, wells, tap water, etc., being the most frequently used water sources in the study area. The radionuclides observed with reliable regularity belonged to the series – decay naturally occurring radionuclides headed by 238U and 232Th as well as the non-series nuclide, 40K. The average specific activity values obtained for 226Ra, 228Ra and 40K, respectively, were 8.67±4.28, 2.31±1.48 and 98.99±6.23 Bq l −1 for well water; 12.45±3.39, 3.02±0.64 and 97.46±6.35 Bq l −1 for borehole water; 12.41±1.37, 2.47±0.09 and 85.06±17.27 Bq l −1 for tap water; 10.40±1.70, 2.70±1.30 and 72.60±9.10 Bq l −1 for dam water; 7.04±0.66, 3.55±0.13 and 69.18±20.80 Bq l −1 for stream water.

Resolution degradation of coaxial p-type germanium detectors due to hole trapping by point defects

A quantitative relationship has been experimentally determined correlation the resolution degradation of p-type coaxial high-purity germanium detectors to the concentration of residual hole-trapping point defects. Detector diameters ranged from 48 to 65 mm, and the average electric-field strength was 1500 V/cm. The concept of a ‘‘standard level’’ is proposed, which originates in the similar capture kinetics of many commonly observed residual acceptors in high-purity germanium. A method for calculating the electric-field dependence of hole capture is presented and used to compare the data reported here with published modeling results parameterized by the mean-free-drift length. Good general agreement is found.

Integrated FET and charge reset device for gamma spectrometers

A specially designed and processed five-terminal device incorporating a low-noise field-effect transistor and an integrated charge-restoration mechanism has been used with an HPGe (high-purity germanium) coaxial detector to produce a high-rate, high-resolution gamma spectrometer. A controlled charge pulse is injected into the FET channel and then collected by the gate to discharge the feedback capacitor and reset the amplifier. The reset time is fast, and the high resolution is maintained at energy rate products in excess of 10/sup 11/ eV/s. The FET input capacitance is 8 pF, and the noise voltage is 0.45 nV/ square root Hz at optimum temperature. When it is used with a 22-pF HPGe n-type coaxial detector, the total pulser noise is 420 eV at 12- mu s amplifier peaking time.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Operation and cross calibration of the activation foil system on TFTR

Time-integrated neutron yields are measured with an activation foil system which pneumatically sends the foils to and from TFTR by remote control. The foils are then counted by high-purity germanium coaxial detectors which have an absolute efficiency calibration. Using different nuclear reactions in different foils with different energy thresholds allows measurements of both DD and DT neutron yields. Details of the system hardware, operation, and efficiency calibration are provided. The cross calibration of the system to other measurements of neutron source strength to provide an absolute measure of yield is described. Comparison of this cross calibration is made to previous absolute calibrations and to neutronics calculations of the response coefficients.

Performance of a Gamma-Ray and X-Ray Spectrometer Utilizing Germanium and Si(Li) Detectors Cooled by a Closed-Cycle Cryogenic Mechanical Refrigerator

A mechanically cooled spectrometer has been constructed and tested separately with Si(Li) planar, high-purity germanium (HPGe) planar, and HPGe coaxial detectors. For each of these types of semiconductor detectors, this spectrometer has attained the lowest noise and best resolution at 5.9 keV, 122 keV, or 1.33 MeV of any closed-cycle mechanically cooled spectrometer which has been reported on in the literature. Resolutions at 5.9 keV were 175 eV FWHM with a 4 mm diameter Si(Li) detector and 202 eV with a 16 mm HPGe planar detector. With a 10% relative efficiency HPGe coaxial detector, resolutions of 816 eV at 122 keV and 1.70 keV at 1.33 MeV were obtained.

The HEAO 3 gamma-ray spectrometer

The Third High Energy Astronomy Observatory (HEAO 3), successfully launched into low earth orbit on 20 September 1979, carries a large high resolution gamma-ray spectrometer designed for cosmic nuclear spectroscopy. This Gamma-Ray Spectrometer (the HEAO C-1 experiment) consists of a cluster of four coaxial high purity germanium detectors, each with a volume of approximately 100 cm 3. Surrounding the germanium detectors is a 6.6 cm thick CsI shield operating in active anticoincidence with the central detectors and defining a field of view of about 30° fwhm. An initial energy resolution of 3 keV fwhm at 1.46 MeV was achieved for each detector. All valid events in the germanium detectors are individually analyzed by an 8192 channel pulse area analyzer and transmitted at a maximum rate of 15.6 events/s for each detector. During a 6 month mission, the experiment will perform a complete sky survey for narrow cosmic gamma-ray line emission to the sensitivity level of about 10 −4 photons/cm 2 s over an operating energy range of 0.05–10 MeV.

Event Timing in High Purity Germanium Coaxial Detectors

The timing of gamma ray radiation in systems using high purity coaxial germanium detectors is analyzed and compared to that of systems using Ge(Li) detectors. The analysis takes into account the effect of the residual impurities on the electric field distribution, and hence on the rate of rise of the electrical pulses delivered to the timing module. Conditions under which the electric field distribution could lead to an improvement in timing performance, are identified. The results of the analysis confirm the experimental results published elsewhere and when compared with those for Ge(Li) detectors, which usually operate under conditions of charge carrier velocity saturation, confirm that high purity germanium detectors need not have inferior timing characteristics. A chart is given to provide a quantitative basis on which the trade-off between the radius of the detector and its time resolution may be made.

N-Type High Purity Germanium Coaxial Detectors

N-Type High Purity Germanium Coaxial Detectors

Radiation Damage Resistance of Reverse Electrode GE Coaxial Detectors

Two high-purity germanium coaxial detectors, having opposite electrode configurations from one another, but fabricated from the same germanium crystal, were irradiated simultaneously with fast neutrons from an unmoderated 252Cf source. Both detectors were 42 mm diam. The detector having the conventional electrode configuration was about 28 times more sensitive to radiation damage than was the detector having the p+ contact on the coaxial periphery. These results prove that germanium coaxial detectors having the conventional electrode configuration should not be used in any situation subject to significant radiation damage. This conclusion was anticipated because the defects produced by neutron and proton irradiation of germanium act predominantly as hole traps.

Timing with high purity germanium coaxial detector

The timing properties of a high purity Ge coaxial detector are studied and compared with those of a Li-drifted detector of similar size. The 1/ r field distribution in a Li-drifted device is modified by fixed space charge in the high purity Ge coaxial detector and this results in a higher and more uniform field distribution in a large portion of the detector volume. The effect is felt particularly in an improved hole collection due to their velocity not being saturated at the 100–500 V/mm fields. It is found that hole mobility obtainable in such fields is reduced by a factor of approximately 8 from the value normally specified at low fields. An immediate advantage of high purity Ge coaxial detectors is found, therefore, in a smaller spread in their charge collection times from which an improved time resolution and smaller energy dependent time walk results, particularly at low energies.