David C. Camp

Finite solid-angle corrections for Ge(Li) detectors

Abstract Finite solid-angle corrections have been calculated for Ge(Li) detectors. Planar detectors considered vary from 4 to 20 cm 2 and from 6 to 16 mm in depletion depth. True circular coaxial detectors vary from 26 to 50 mm in diameter with depletion depths from 7 to 20 mm and lengths from 22 to 57 mm. Five-sided detectors of both trapezoidal and circular cross sections are included, and these range from 20 to 70 mm in length and from 8 to 20 mm in depletion depths. The corrections are calculated for source distances of 3.0, 5.0, 7.0 and 10.0 cm. Results are presented in tables rather than as graphs for greater accuracy. Interpolation is easy and accurate for configurations and distances other than those given. Finite size sources cause small reductions in the point source corrections. Graphs and formulas are presented which allow the reductions to be calculated for spherical or cylindrical sources.

Low-background Ge(Li) detector systems for radioenvironmental studies

Abstract Two Ge(Li) detector systems designed to measure low-level radioenvironmental samples are described. The materials used in the construction of these systems and the effect of these materials on the background radioactivities are discussed. One system, designed to count mid-range specific activities, contains a closed, coaxial, 10% Ge(Li) detector that is housed in electrolytic magnesium and operated within a standard lead cave. Continuum background levels in this system vary from 0.1 c/min keV at 200 keV to 0.01 c/min keV at 1 MeV. The second system, designed for maximum sensitivity, contains a 15%-relative-efficiency Ge(Li) detector housed in electrolytic magnesium and operated in anticoincidence with a large plastic detector. This entire system is placed inside a lead-and steel armor plate shield located in a counting room below ground level. For single gamma-ray emitters from point sources, the maximum continuum suppression factor is greater than 8. Under suppression conditions the background continuum, as well as the usual contaminant peaks, are the lowest yet reported for Ge(Li) environmental counting systems. Continuum values decrease from about 6 × 10 −3 c/min keV at 200 keV to less than 3 × 10 −4 c/min keV at 1 MeV. A 1000 min count of 250 g of soil shows that this system can measure a 137 Cs specific activity of 20 fCi/g with a precision of 15%. The lowest specific activity per gram detectable in a one-week count under threshold conditions (± 50%) for a single gamma-ray emitter with E γ >500 keV is several hundred aCi/g.

Energy levels in 72Ge from the decays of 72Ga and 72As

Abstract The γ-ray decay scheme and nuclear energy levels in 72 Ge have been studied from the decays of 72 Ga and 72 As using a high-resolution 7 cm 3 Ge(Li) detector as the central detector in a Compton suppression spectrometer and using NaI(Tl)-Ge(Li) coincidence data. A discussion of all experimental variables encountered in reducing the Ge(Li) detector data is included. Careful hand and computer analyses determined as accurately as possible the energies and intensities of 122 separate γ-ray transitions; 85 of them observed in 72 Ga and 85 in 72 As. Of the total number of γ-rays observed, 106 have been placed in a proposed level scheme containing 32 levels. These include 20 new levels, six of which are tentative. The end-point energies of the various β-transitions, their branching ratios and log ft values are deduced from the γ-ray branching ratios.

Delay Line Readouts for High Purity Germanium Medical Imaging Cameras

High purity germanium offers excellent potential for use in nuclear medical imaging cameras. A position and energy readout technique using two inexpensive delay lines has been developed for these cameras. Results obtained with a 1-cm2 , 4 mm deep, 5×5 strip high purity germanium detector are 2.1 mm full width spatial resolution, a measured single strip resolution of 0.65 mm full width half maximum (FWHM), a 25 element uncollimated energy resolution of 2.95 keV FWHM, and 2.65 keV FWHM for a single central element at 140 keV.

Two-Detector, 512-Element High Purity Germanium Camera Prototype

A gamma-ray camera consisting of two 3.2×3.2×1-cm3 HPGe detectors has been assembled. Shallow orthogonal grooves define 512 2×2-mm2 elements. Square hole collimators have been fabricated with design parameters that exploit the unique characteristics of the detector. Intrinsic spatial resolution is a square function with 2-mm width, and energy resolution is approximately 2.5% FWHM at 140 keV. Evidently superior images are obtained when this instrument is compared to state-of-the-art scintillation cameras.

Energy levels in 76Ge from the decay of 76Ga

Abstract The γ-ray decay scheme and nuclear energy levels in 76 Ge have been investigated from the decay of 76 Ga using a high-resolution 38 cm 3 Ge(Li) detector. The half-life of 76 Ga was determined to be 27.1±0.2 sec. Energies and intensities are given for 107 γ-ray transitions of which 97 are placed in a proposed level scheme containing 25 definitive and 16 tentative levels. Log ft values are deduced from the relative γ-ray intensities. Spin and parity assignments are proposed for some of the levels.

CORRECTIONS TO COAXIAL Ge(Li) DETECTOR SOLID ANGLE CORRECTION FACTORS.

Abstract This letter discusses additional corrections needed for solid angle correction factors for coaxial detectors.

Energy levels in 74Ge from the decay of 74Ga

Abstract The γ-ray decay scheme and nuclear energy levels in 74 Ge have been studied from the decay of 74 Ga using a high-resolution 38 cm 3 Ge(Li) detector. The half-life of 74 Ga was determined to be 8.25 ± 0.05 min using chemically separated sources. Energies and intensities are given for 97γ-ray transitions of which 77 are placed in proposed level scheme containing 29 levels. Log ft values are deduced from the relative γ-ray intensities, and spin and parity values are proposed for some of the levels.

An intercomparison of results from samplers used in the determination of aerosol composition

Abstract An EPA sponsored field study intercompared the performance of 11 different aerosol samplers. They were operated simultaneously for eight consecutive days by scientific groups having expertise in sampler development and operation. Collection surfaces were changed every 12 h allowing a 16 period intercomparison. Samples were analyzed at each participants laboratory for one or more of the pollutants mass, nitrate, sulfur or sulfate, lead, and 8 other elements using one or more analysis methods. Most samplers collected two particle-size fractions with the 50% separation for the fine fraction between 2.4 and 4.3 μm. Upper cutoff diameters ranged from 14 to above 30 μm which strongly influenced the total mass collected. Results were intercompared for the total, fine, and coarse size fractions. Agreement to better than 10% was found for the elements Zn, S, and Pb that occur primarily in the fine fraction. Mean sulfate concentration was three times the sulfur concentration, showing that all sulfur occurs as sulfate.

A High Intensity Source of Polarized X-Rays for Fluorescent Excitation Analysis (FEA)

FEA is being used in medicine to substitute radiotracers and chemical analysis of stable tracers. Excitation with polarized x-rays reduces tracer working levels: Since they scatter at preferential angles they reduce the number of photons reaching the detector, thus decreasing system dead time and background. The use of a 160KV 19mA x-ray tube for FEA of iodine (and neighboring elements) realizes count-rates of 2.5 cts/sec/ppmI and background reductions from 11.5 ppmI (when using Am-241 excitation) to 5.7 ppmI. These early results indicate that counting time to realize constant quantitation accuracy can be reduced by 90%. Since the use of the x-ray tube results in a 50% increase in the cost of the source-excited automated system, the ten-fold reduction in analysis time makes this a practical approach.