J.M.R. Hutchinson

Backscattering of alpha particles from thick metal backings as a function of atomic weight

Abstract Alpha-particle backscattering from thick metal backings has been studied using two separate counters with geometries of 2 π and approximately 1 π steradians.

Study of the scattering correction for thick uranium-oxide and other α-particle sources—II: Experimental

Abstract Theoretical values of the 2π α-particle counting rate, C2π, including scattering, divided by the disintegration rate, N0, are calculated for α-particle sources mounted on flat backing materials. The theoretical values (Part I) and experimental values (Part II) are in good agreement, and show that 1. (a) C2π/N0 values as a function of α-particle energy and source thickness can be calculated on the basis of a simple physical model. 2. (b) The scattering correction, expressed as a fraction of the disintegration rate, decreases with increasing source thickness. 3. (c) The α-particle scattering in uranium-oxide is much less than the scattering in platinum, in disagreement with previous estimates. The calculations are readily extended to other source compositions and α-particle energies.

“Pin-Well”-NaI(Tl) counting of 59.5-keV γ-rays in the decay of 241Am

Abstract The 59.5-keV γ-ray-emission rate in the decay of an 241 Am source has been measured using a specially constructed NaI(Tl) thin-walled-well detector. From this measurement, and an α-particle-emission rate measurement on quantitatively related sources, the 59.5-keV γ-ray probability per decay is 0.3582 ± 0.0012.

Standardization of cadmium-109 sources for γ-ray emission rate

Abstract The standardization of 109 Cd sources for 88-keV γ-ray emission rate has been performed using a NaI(Tl) well crystal which gives a nearly 4π geometry. Since the low-energy portion of the 88-keV γ-ray spectrum is obscured by 22-keV X-ray pulses, the total counting rate cannot be found by merely summing the counting rates for all the channels in the distribution. A method, in which the spectrum is broken down into eight components, is used to determine the number of degraded 88-keV γ-ray pulses. The calibration is performed by summing the counting rates of the various components below an arbitrarily chosen energy of 39 keV, and adding them to the total counting rate above 39 keV. The process in which photoelectrons escape into the aluminum liner of the well is found to be responsible for most of the degraded 88-keV γ-ray pulses which fall below 39 keV. The estimated systematic error in the measurement is ±0·9 per cent. Using 57 Co sources which have been accurately calibrated by coincidence counting, the 2-in. × 2-in. NaI(Tl) well crystal is shown to have a γ-ray detection efficiency of 100 per cent with an uncertainty of ±0·2 per cent (after corrections have been made for well-wall γ-ray absorption and excape out of the well). The total abundance for the 122- and 136-keV 57 Fe photons emitted per decay from 57 Co has been measured to be 0·963 ± 0·003.

Needs for radioactivity standards and measurements in different fields

Abstract With the enormous and world-wide increase in the applications of radioactive materials in so many scientific and technological fields, a corresponding need has arisen for increased skill and improved facilities for the assay of such materials. In the U.S.A., the use of radioisotopes, including radiopharmaceuticals is about

Determination of source self-absorption in the standardization of electron-capturing radionuclides

80 million annually. The measurements skills and facilities must of necessity tend to lag behind such a rapid growth, and increasingly from many quarters we at the National Bureau of Standards are being requested for help in establishing a consistent measurements system in radioactivity traceable both to us and through us to the international measurements system. This paper will discuss the measures that we are taking in order to achieve such traceability in a consistent measurements system in different fields of radioactivity, with special emphasis on environmental and radiopharmaceutical measurements.

A bone ash standard for90Sr,210Pb,210Po, uranium and the actinides

Abstract A method is described for measuring source self-absorption in the standardization of radionuclides which decay by electron capture directly to the ground state. Auger electrons are used as a tracer in order to determine the X-ray absorption. Data are presented which indicate that X-ray absorption in a certain type of electroplated source is less than 0.2 per cent. A method for determination of ejected photoelectrons from absorbing foils in 4π X-ray counting is described.

The half lives of two excited states in 239Np

A bone ash standard for a number of radionuclides is required as a quality control sample in dosimetry studies with bone as the critical organ. The procedures that were used to prepare a candidate bone ash standard are given with some initial encouraging measurements that the candidate sample will meet the requirements of NIST standard reference manual.

Calibration of Ge(Li) gamma-ray spectrometers for the measurement of radioactive noble gases

The half-life of the 74 keV (52-) state of 239Np has been determined to be 1·380 nsec ± 0·032 nsec where the uncertainty represents the sum of estimated bounds to the systematic error, ±0·010 nsec, and twice the standard error, ±0·022 nsec. The half-life of the 118 keV (72-) state of 239Np has been found to be less than 0·04 nsec.

Calibration of K-x-ray-emission rates in the decay of 49V

Abstract Assays of radioactive noble gases, in particular reactor gaseous effluents, are routinely made with Ge(Li) gamma-ray spectrometers.(1) In the past, significant uncertainties in these assays have been associated with the calibration of the detector, that is, with the preparation of a curve showing full-energy-peak efficiency as a function of gamma-ray energy. A method of preparing such a curve, based on measurements made with gamma-ray point-source standards, is given here. An expression is given that relates the full-energy-peak efficiency for a point source, centered above a glass hemisphere, to that of a gaseous source in a spherical ampoule, for a given gamma-ray energy. Measurements made with standards of krypton-85, xenon-127, xenon-131m, and xenon-133 demonstrate that the method is suitable for the intended purpose.