Spectrometry of a⁶⁰Co Gamma-Ray Beam Used for Instrument Calibration

Abstract

Measurements were made of the spectrum of one of the collimated 60Co gamma-ray beams employed in the instrument-calibration program of the National Bureau of Standards. The high-activity calibration-source capsule was replaced by one of low activity and identical geometry. The contributions to the spectrum by the scatter from the low-activity replacement source, the housing, and the collimation system were isolated by means of supplementary measurements in free air carried out on the replacement source and on a scatter-free source. Depending on collimator setting, the scatter contribution below 1 MeV was found to be between 13 and 19 percent of the total number of photons in the beam. A description is given of the measurements and of the technique for data reduction. Detailed spectral results are presented.

Exposure spectra from NBS vertical-beam ⁶⁰Co gamma-ray source

“…The results are presented in tables 1 This contribution is entirely due to scattered photons. The uncertainty attached to its value represents one standard deviation, compounded from the statistical error in the count rate, the unfolding error, and the goodness of fit between the unfolded spectra and the measured pulseheight distributions [4].…”

“…Procedures for arriving at spectral distributions from the experimental pulse-height distributions obtained at two distances from the source, and for isolating the spectra due to scatter from the source proper and from the housing and collimator were reported elsewhere [1]. In practical applications to instrument calibration, one usually is interested not in the number of scattered photons in a particular photon-energy interval but in the contribution of these photons to the total exposure --information that can be derived readily from the photon spectra.…”

“…It is the purpose of this report to present, in tabular form, such exposure spectra for the scattered radiation below 0. the corresponding mass energy-absorption coefficient of air [2], and the total exposure, X, is given by Because of experimental evidence that the scatter contribution --beyond a certain lower distance limit --changes only slowly with distance from the source [1], it is justified to take the experimental scatter spectra obtained at a distance of 1.08 m to be identical, for all practical purposes, with scatter spectra at a distance of 1.00 m.…”

“…The exposure spectra were computed from the measured number spectra [1] as outlined, and smooth curves were drawn of exposure contribution in each energy interval as a function of field size used in the measurements. The exposure contribution for the field sizes and the distance of interest were determined by interpolation of these curves.…”

On-Site Calibration Method of Dosimeter Based on X-Ray Source

“…In order to further verify the energy response value after experiment correction, the experiment of energy response using 137 Cs source was carried out at Northwest Institute of nuclear technology [17], and the results are shown in Table 5.…”