Pu-Be Neutron Source Research Articles

Recoil Gold Atoms from Fast-Neutron Bombardment

Gold foils were irradiated with fast neutrons from a Pu-Be neutron source and the gold atoms knocked out of these foils were collected on aluminum foils. Sets of Au-Al foils were placed at various angles around the Pu-Be source. Some foil sets had the gold foil nearer the neutron source; some had the aluminum foil nearer. In this way, both the ejection of gold atoms from open-ended displacement spikes and the sputtering of gold atoms by fast neutrons could be studied. As was expected, no 198Au was detected on the Al foils because of the small (n,γ) cross section of 197Au. After neutron activation in a nuclear reactor, all the Al foils emitted the 0.412-MeV gamma ray of 198Au. Calculations showed that each Al foil had approximately 1012 197Au atoms on it. Since the Pu-Be neutron source strength was known, the number of gold atoms ejected per open-ended displacement spike was calculated and found to be of the order of magnitude of 106. Focusing collisions may account for the large number of Au atoms per open-ended displacement spike. The sputtering ratio was found to be about 0.5 gold atoms sputtered per incident neutron.

An Absolute Thermal-Neutron-Flux Standard

A useful and inexpensive assembly for thermal-neutron-flux standardization is described. This assembly consists of a water filled tank containing three Pu-Be neutron sources, each located at an apex of an equilateral triangle with sides of 5.6 inches and appropriately centered in the tank. The thermal flux is spatially invariant and isotropic over a rather large region in the test section and thus reduces the effects of positioning errors of foils and detectors being calibrated. Methods for standardizing such an assembly are discussed.

The Stilbene Scintillation Crystal as a Spectrometer for Continuous Fast-Neutron Spectra

A method is described for measuring neutron spectra in the energy range 1 to 10 Mev. The pulse-height spectrum of scintillations produced in a stilbene crystal by recoil protons originating in the crystal is analyzed to yield the incident neutron energy distribution. Insensitivity to gamma rays and electrons is achieved by utilizing differences in the pulse shapes of light pulses produced by electrons and protons. This spectrometer is useful for both group and continuous spectra and has been tested by measurement of the spectra from a Pu-Be neutron source and from the spontaneous fission of Cf252. Both measurements are consistent with previous data. The Pu-Be data definitely confirm a peak at 9.7 Mev that was suggested by previous data.

Determination of the Quality of the Absorbed Dose Delivered by Monoenergetic Neutrons

In order to allow a more precise characterization of the quality of ionizing radiation, a spherical proportional counter has been developed at this laboratory. It was shown (1) that because of its shape the pulse-height spectrum obtained with this counter could under certain conditions be subjected to a mathematical analysis in order to obtain the distribution of the dose in the LET (linear energy transfer) of the ionizing particles. Actual measurements on Pu-Be and Po-B neutron sources and on 3-Mev neutrons from a deuterium (d, n) reaction were performed and reported (2). At that time a comparison was made with calculations by Boag (3) based on the LET versus energy curve for protons in water. Some differences between the calculated and observed spectra were found and possible causes discussed. Subsequent measurements indicated that the differences could not be completely explained on the basis of experimental error and that the measured spectrum must differ from the calculated one because of physical factors that are more fundamental. The calculated spectrum uses the concept of the LET of a particle at a point. This is a useful abstraction which can never be quite approached in a physical measurement. The gas volume of the counter must take a finite sample from the track of the ionizing particle. If the sample is very small there will be large statistical fluctuations in the energy loss of the particle owing to the discontinuous nature of the collision process and the possible loss of energetic 5-rays to the counter wall. This effect will be minimized by making the sample larger, but in this case the ionizing particle loses so much energy that it can no longer be characterized by a single LET. It has already been pointed out (4) that the counter measurement presents a picture which resembles more closely events that take place in a sphere of tissue under irradiation. The present work is a study of the pulse-height spectra and resultant dose distributions for monoenergetic neutrons as a function of neutron energy and also sample size. The neutrons were produced at a Van de Graaff generator at the follow

Neutron Spectrum and Absolute Yield of a Plutonium-Beryllium Source

A Pu-Be neutron source has the advantages of high neutron yield, low gamma-ray intensity, and very long half-life. The neutron spectrum from such a source, composed of 13 g of plutonium and 7 g of beryllium as Pu${\mathrm{Be}}_{13}$ (density 3.7 g/cc), was measured by means of proton recoils in nuclear research emulsions. The data obtained from 2057 observations indicate the neutrons have a maximum energy of approximately 10.5 Mev with broad intensity maxima at 4.0, 7.2, and 9.7 Mev. The observed spectrum shows marked similarities with published data on neutrons from Po-Be and Ra-Be sources. A comparison is made between the present data and previously reported energy levels in the residual carbon nucleus. The absolute yield of neutrons above 0.5-Mev energy is 1.2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ neutrons/sec.