V. E. Iacob

Precise efficiency calibration of an HPGe detector: source measurements and Monte Carlo calculations with sub-percent precision

With the goal of measuring precise gamma-ray intensities for short-lived (< 5 s) accelerator-produced activities, we have calibrated the efficiency of an HPGe detector between 53 and 1836keV to sub-percent precision with a combination of source measurements and Monte Carlo calculations. Using known or independently measured detector dimensions, we have achieved both relative and absolute agreement (the latter, to 0.1%) between the calculated and measured efficiencies with only two adjustable detector parameters, the thicknesses of the contact dead layers.

Production of Co-60 sources for high-accuracy efficiency calibrations of gamma-ray spectrometers.

Activity determination by gamma-ray spectrometry is a valuable aid which is widely used where radioactive sources are applied. For the construction of an efficiency curve vs. energy in a standard geometry, the accuracy is limited not only by the uncertainty of peak area determination and the knowledge of the relevant emission probabilities but also by the uncertainties of the activity values applied to efficiency calibration with standard sources. Recently, a method to determine a relative efficiency function using selected radionuclides with gamma cascades of well known emission probabilities has been developed which yields very accurate efficiency values. Such a curve has to be converted into an absolute efficiency curve by calibration of the detector system with at least one source of very well known activity. Especially for this purpose, a set of Co-60 gamma-ray spectrometer sources was produced at the PTB. The activity values (10-45 kBq) were measured by the 4pi beta-gamma coincidence technique with a standard uncertainty of less than 0.1%. Production and measurement of these sources are described in the present paper. Special care was taken to include all contributions to the standard uncertainty of the activity value when establishing the uncertainty budget for the activity of each of these sources. The activity ratio of two of these sources, as obtained by the absolute measurements, was checked by independent gamma-ray counting at two other laboratories, where the precision achieved was less than 0.2%. The procedure described is considered to be a step towards more accurately determined efficiency functions in gamma-ray spectrometry.

Do radioactive half-lives vary with the Earth-to-Sun distance?

Recently, Jenkins, Fischbach and collaborators have claimed evidence that radionuclide half-lives vary systematically over a ±0.1% range as a function of the oscillating distance between the Earth and the Sun, based on multi-year activity measurements. We have avoided the time-dependent instabilities to which such measurements are susceptible by directly measuring the half-life of (198)Au (t(1/2)=2.695 d) on seven occasions spread out in time to cover the complete range of Earth-Sun distances. We observe no systematic oscillations in half-life and can set an upper limit on their amplitude of ±0.02%.

Precise half-life measurement of the superallowed beta(+) emitter (26)Si

We have measured the half-life of the superallowed 0+ -to- 0+ beta+ emitter 26Si to be 2245.3(7) ms. We used pure sources of 26Si and employed a high-efficiency gas counter, which was sensitive to positrons from both this nuclide and its daughter 26mAl. The data were analyzed as a linked parent-daughter decay. To contribute meaningfully to any test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the ft value of a superallowed transition must be determined to a precision of 0.1% or better. With a precision of 0.03% the present result is more than sufficient to be compatable with that requirement. Only the branching ratio now remains to be measured precisely before a +/-0.1% ft value can be obtained for the superallowed transition from 26Si.

Study of excited states of {sup 31}S through beta-decay of {sup 31}Cl for nucleosynthesis in ONe novae

We have produced an intense and pure beam of 31Cl with the MARS Separator at the Texas A&M University and studied β‐decay of 31Cl by implanting the beam into a novel detector setup, capable of measuring β‐delayed protons and γ‐rays simultaneously. From our data, we have established decay scheme of 31Cl, found resonance energies with 1 keV precision, have measured its half‐life with under 1% accuracy, found its Isobar Analog State decay and by using the IMME obtained an improved mass excess for its ground state. In this contribution, a description of the used method along with selected preliminary experimental results are given and their relevance for novae nucleosynthesis discussed.

Precise tests of internal-conversion theory

Using the 165.9-keV M1 transition in (139)La as a calibrant, we have determined the K-shell internal conversion coefficients (ICCs) for the 127.5-keV E3 transition in (134)Cs and the 661.7-keV M4 transition in (137)Ba to 0.6% precision. Unlike previous less-precise measurements of these ICCs, which differed significantly from calculated values, our results agree well with calculations that include the atomic K-shell vacancy and disagree with those that exclude it. These results, which are consistent with what we also found for (193)Ir, confirm our earlier conclusions but for different energies, multipolarities and atomic numbers.

Astrophysical Reaction Rates Obtained By Indirect Techniques

Indirect techniques have been used to obtain information about reaction rates for several proton capture reactions that occur on short‐lived nuclei. The techniques used to carry out the measurements are reviewed and the results obtained are presented. Also future prospects for further measurements with a new facility, T‐REX are discussed.

β‐decay of 23Al and nova nucleosynthesis

We have studied the β‐decay of 23Al with a novel detector setup at the focal plane of the MARS separator at the Texas A&M University to resolve existing controversies about the proton intensities of the IAS in 23Mg and to determine the absolute proton branching ratios by combining our results to the latest γ‐decay data. Experimental technique, results and the relevance for nova nucleosynthesis are discussed.

Precise half-life measurement of the superallowed beta(+) emitter (10)C

We measured the half-life of the superallowed 0{sup +{yields}}0{sup +} {beta}{sup +} emitter {sup 26}Si to be 2245.3(7) ms. We used pure sources of {sup 26}Si and employed a high-efficiency gas counter, which was sensitive to positrons from both this nuclide and its daughter {sup 26}Al{sup m}. The data were analyzed as a linked parent-daughter decay. To contribute meaningfully to any test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the ft value of a superallowed transition must be determined to a precision of 0.1% or better. With a precision of 0.03%, the present result is more than sufficient to be compatible with that requirement. Only the branching ratio now remains to be measured precisely before a {+-}0.1% ft value can be obtained for the superallowed transition from {sup 26}Si.

SU‐GG‐T‐240: New Method of An HPGe Detector Precise Efficiency Calibration with Experimental Measurements and Monte Carlo Simulations

Purpose: Developing a method of an HPGe detector precise γ efficiency calibration which is very important for accurate radiation detection during cancerradiotherapy practices. Method and Materials: 24 Al radioactive nucleus produced and separated with Momentum Achromat Recoil Spectrometer (MARS) at the K500 superconducting cyclotron of Texas A&M University has positron decays followed by γ transitions up to 8 MeV from 24 Mg excited states, which is used for a β‐γ coincidence measurement with a 1‐mm‐thick BC404 plastic scintillator, an HPGe detector and a fast tape‐transport system to calibrate the HPGe detector.Results: By carefully considering the effects of summing, positron annihilation, internal conversion, and β detector efficiency when analyzing 24 Al spectrum, we got the efficiency for γ‐ray 7070 keV at 49 mm distance away from the source sample 24 Al , which was 0.192(6)%. The Monte Carlo(MC) simulation with CYLTRAN code gave a value of 0.189%, which was in agreement with our measurements. The precise efficiency calibration curve of the HPGe detector up to 7070 KeV at 49 mm distance away from the source sample was obtained. By using the same procedure, we got the efficiency for the 7070 keV γ‐ray at 151 mm distance away from the source sample 24 Al , which was 0.0385(8)%. MC simulation value was 0.0399%, which differed from measurement by 4(2)%. This discrepancy led us to assign an uncertainty of 4% to our efficiencies at 151 mm up to 7070 KeV. The Monte Carlo calculations also reproduced the intensity of observed single‐and double‐escape peaks, providing that the effects of positron annihilation‐in‐flight were incorporated. Conclusion: A new method was established. The precise calibration curves obtained from this work are useful for accurate radiation detection and improving quality control to quality assurance (QA) for intensity‐modulated radiation therapy (IMRT). Research sponsored by Department of Energy and Robert Welch Foundation.