J. M. Palms

A Ge(Li) Concentric Duode Spectrometer for Compton Suppression

A Ge(Li) Compton sum coincidence spectrometer which greatly reduces the Compton continuum and essentially eliminates the Compton edges for low energy gammas has been fabricated. This spectrometer was made from a single cylinder of germanium in which lithium was drifted to form two separate and equal depth concentric detectors. An event in the full energy peak is recorded only if the total energy deposition occurs as the result of multiple events, with each detector sharing in the absorption process. Several types of gamma-ray spectra are presented which show the reduction of the Compton continuum. For the case of the 1275 keV gamma from 22Na the ratio of the full energy peak to Compton edge (in this case a multiple Compton event edge) increases from 2.8 to 1, when the device is used as the sum of two single spectrometers, to 13.5 to 1 when it is used in the sum coincidence mode with a decrease of efficiency of a factor of five. This ratio for the 511 keV line from 22Na increases from 3.6 to 1, to 18 to 1. Other spectra are shown and a discussion of efficiency is presented.

Eye Tumor Identification Tests Using Semiconductor Detectors

The 32P eye tumor identification test is being reevaluated using semiconductor detectors. Diagnostic tests for both anterior and posterior lesions of the uveal tract are being performed on patients, and a comparison is being made between the conventional G. M. eye tumor counter and the silicon lithium drifted, Si(Li), silicon surface barrier and silicon avalanche beta detectors. Results from 31 tests performed to date indicate that among the detectors tested, the Si(Li) posterior detector gives the most consistent, sensitive and reliable results. For tumors of the uveal tract a beta energy threshold as determined by the detector window thickness and electronic noise can be as high as several hundred keV. Surface barrier detectors placed in suitable probes can therefore also be used for these lesions. For more deeply imbedded tissue tumors, such as lesions of the choroid, iris or retina, the silicon avalanche detector shows considerable promise because of its greater sensitivity for lower energy betas.

Medical Problems in Eye Tumor Identification

The accurate identification of eye tumors continues to be a difficult medical diagnostic problem. The several non-nuclear clinical diagnostic techniques that have been used, such as direct and indirect ophthalmoscopy, slit lamp examination, transillumination, fluorescein angiograph, ultrasonography and direct analysis of the subretinal fluid, have all resulted in a high percentage of misdiagnoses. This has resulted in eyes being enucleated that did not contain malignant melanomas, but more tragically, eyes with malignant tumors were not enucleated, resulting in the death of the patient due to the metastatic spread of the tumor. At the present state of development, the 32p uptake test offers the best diagnostic method. For tumors of the uveal tract both G. M. and semiconductor detectors can be used. Detectors more sensitive to lower energy betas are needed to diagnose deeper lying tumors and to study the possible correlation between percent 32p uptake and the particular cell type of the tumor.

A Gamma-Ray Spectrum Analysis Technique for Low-Level Environmental Radionuclides

The requirements for gamma-ray spectra analysis for low-level environmental radionuclides are reviewed. Special emphasis is placed on the routine and rapid analysis of large numbers of samples. Pertinent requirements for analysis include the system energy and efficiency calibration, the library of gamma rays of concern, criteria of establishing the limits of sensitivity and techniques used for gamma-ray intensity and background determinations. An analysis code (CETUS) used with a Ge(Li) detector and multichannel analyzer interfaced to a programmable desk-top calculator is described.

K-electron capture probabilities in the decays of Gd151 and Dy159

TheK-electron capture probability,PK, in the allowed and non-unique first-forbidden transitions is measured by observing coincidences betweenK x rays and γ rays. The following results are obtained. In Gd151 decayPK to the 349.8 keV level is 0.713 ± 0.015 andPK to the 307.4 keV level is 0.813 ± 0.020. It is also deduced that the decay energyQEC is 491−9+14 keV and the spin of the 307.4 keV level is 7/2. In Dy159 decayPK to the 58.0 keV level is found to be 0.803 ± 0.032.

Cadmium telluride for low energy gamma-ray and X-ray spectrometers

Abstract Cadmium telluride detectors have been fabricated which are useable as low energy gamma and X-ray spectrometers. A threshold energy of 10 keV and a resolution of 9.8 keV (fwhm) were obtained for the 59 keV line from 241 Am. K X-ray escape peaks for Cd and Te have been observed.

Electron capture decay of181W: L subshell fluorescence and Coster-Kronig yields in Ta

The TaL x-ray spectrum from the 140 d electron capture decay of181W was studied with high resolution semiconductor detectors and fast coincidence techniques. Measurement ofL x-ray-K x-ray andL x-ray—γ-ray coincidence rates yielded the followingL shell fluorescence and Coster-Kronig yields and radiative decay branching ratios: ω2=0.250±0.013, ω3=0.228±0.013 (corrected for angular correlation effects),f23=0.180±0.007,v1=O.218±0.016,s3=0.205+-0.010,s2=0.215+-0.010,f13+f12f23=0.36+-0.02, and ω1+f12ω2=0.14±0.02, from which upper limits were obtained off13<0.36 and ω1<0.14.

Decay of194Os andL1 subshell yields atZ=77

The relative intensities of x rays and gammas emitted in the beta decay of194Os are measured. No evidence for the feeding of a level at 83 keV in194Ir is found. An upper limit of 1.7×10−4 was set for theK shell internal ionization probability in the beta decay to the ground state of194Ir. TheL-conversion coefficient of 43 keV transition is found to be 12.1 and the transition is mainly ofM1 type with an admixture of 1.3%E2 type. TheL1 subshell yields atZ=77 are determined to beω1=0.16±0.04,f12=0.11±0.04 andf13=0.37±0.03.

A Baseline Ground-Level Airborne Radionuclide Survey for a Nuclear Fuel Reprocessing Plant

The ground-level airborne radionuclide concentrations and the resulting dose to man have been measured in the vicinity of Barnwell, S.C., during 1971, 1972, and 1973. These measurements, which were made as part of a preoperational baseline study for a nuclear fuel reprocessing plant, include gross alpha-beta and high resolution Ge(Li)-NaI(Tl) gamma-ray spectrometry using air filters, tritium (HTO) measurements using silica gel, and dose measurements using LiF thermoluminescence dosimeters. The observed levels of approximately 20 nuclides can be explained by contributions from worldwide fallout, natural process and naturally occurring terrestrial nuclei, and normal seasonal variations. There has been a considerable decline in levels during the latter part of 1972 and first half of 1973. The average annual dose to man resulting from inhalation of most of the measured nuclei is only a very small fraction of the total dose received from normal background sources.