Anticoincidence Shield Research Articles

Total Absorption Gamma-Ray Spectrometers Utilizing Anticoincidence Shielding

Two total absorption gamma-ray spectrometers utilizing anticoincidence shielding are described. One of these consists of a 5-in. diam by 5-in. thick NaI(Tl) well crystal enclosed in a 26-in. diam by 30-in. high plastic phosphor. The plastic phosphor serves as the anticoincidence guard. The second spectrometer consists of a 3-in. diam by 3-in. thick NaI(Tl) crystal with a 9⅜-in. diam by 8½-in. thick NaI(Tl) well crystal as its anticoincidence guard. With the anticoincidence shielding, and a point source in the well of the 5×5-in. crystal, the Compton portion of the Zn65 spectrum is reduced by a factor of about 5, while the Compton portion of the Co60 spectrum (below 1 Mev) is reduced by factors of 20 to 25. With these point sources on top of the 3×3-in. solid crystal, these Compton regions are reduced by factors of about 4 and 14, respectively. These anticoincidence shielding arrangements also provide background reductions of a factor of 2 to 5 in the energy region below 3 Mev. The response of these spectrometers to other gamma-ray energies is included. The photomultiplier mounting arrangements for the large NaI(Tl) and plastic phosphors, and the resolutions of the plastic and NaI(Tl) crystal are discussed.

An Improved Anti-Coincidence Shield For Use In Low Background Counting

A liquid scintillator is used as an anticoincidence shield for eliminating the cosmic ray component of the background of a Geiger-Muller (G.E.C. Type GM 4 LB) counter. The apparatus is described and its performance compared with that of a shield of Geiger counters. Areduction inbackground from approximately 0.8 cpm (as measured with the Geiger shield) to approximately 0.45 cpm, as measured with the scintillator shield, was obtained. Evidence is presented which suggests that the reduction is due to the detection of cosmic ray induced reactions in the material surrounding the counter.

A Low Background Beta Counter with a Plastic Scintillator Anticoincidence Shield

The design and performance of a low background beta counter is described. The counter consists of a central gas flow proportional counter surrounded by a large plastic scintillator anticoincidence shield.

Application of bidirectional dekatron tubes in low level beta activity measurements by cancellation technique

A new approach employing cancellation technique for reducing background of G.M. counter (size: diameter = 2.5 cm, length = 5 cm) to less than 2 counts per minute has been developed for the measurement of low level beta activity of the order of 25 micromicrocuries. The method and electronics used for cancellation are described. This method is a simple alternative to the usual anticoincidence shielding and needs only two G.M. counters for determination of moderately low level beta activities. This technique has been successfully applied for the study of the activity of biological samples from sea.

Low Background Thin-Wall Flow Counters for Measuring Beta Activity of Solids

The construction and performance of disk-shaped thin-walled flow counters suitable for measuring very small beta activities of solids is described. The circular faces, which serve as the cathode, consist of conducting films. Either a single pear-shaped loop of wire whose plane is parallel to that of the faces, or a group of straight wires stretched across the midplane serves as the anode. The counters are operated in the Geiger region. Samples can be counted against both faces of the counter. Performance data are given for counters having active volumes ranging from 0.2 to 7 cc, and having active areas from 1.6 to 23 cm2. Counting efficiencies for natural potassium (mounted on Lucite) vary from 30% to 47%, depending on the relationship of sample area to counter dimensions. Inside a 20-cm steel shield and an anticoincidence ring the background counting rates varied approximately as 1 count/hr/cm2 of counting area. Backgrounds as low as 1 count/hr were obtained. The disk geometry permits the use of a smaller metal and anticoincidence shield than is possible with cylindrical geometry. Counters shielded successively with 1.9-cm mercury, 5-cm steel, and 5-cm lead, and operated with a flat multiple-anode anticoincidence counter (15 cm2), actually gave lower background than with the 20-cm steel and anticoincidence ring assembly. An integral counter assembly, consisting of ∼2-cm diam disk-shaped counter and a disk-shaped guard counter (∼7-cm diam), was constructed and gave satisfactory results.

Large Crystal Total Absorption Spectrometer

A total absorption scintillation spectrometer has been built primarily for the study of the neutral component of cosmic radiation. The high energies involved necessitate a large NaI(Tl) crystal (∼70 lb) completely surrounded by an efficient anticoincidence shield. The design and performance of this instrument are discussed.

Radiocarbon dating and post-glacial vegetational history: Scaleby Moss

A proportional gas-counter of about 2 1. volume, within a castle of 5 tons of zinc, enclosing in turn an anticoincidence shield, has been used to make radiocarbon assays of highly purified carbon dioxide at a pressure of 2 atm. A net contemporary count-rate of twenty-seven counts per minute has been secured with a constant background rate of twenty-five. The chief features of this apparatus and of the combustion technique are briefly indicated. The radiocarbon assays have been employed to provide age determinations from a series of peat samples secured from a dry peat-bog at Scaleby Moss, Cumberland. The whole peat sequence has been pollen-analyzed and each radiocarbon sample is very exactly referred to a major zone boundary in a zone sequence which extends from the Late-glacial period continuously through almost the whole of the Post-glacial period. The dates obtained have been based upon an estimated half-life for radiocarbon of 5568 ± 30 years. The errors of determination are of the order (1 standard deviation) of ± 150 years. The results are satisfactorily self-consistent, contiguous samples have closely similar dates, and the whole series of dates follows the stratigraphical sequence. It is shown that at Scaleby Moss the Late-glacial/Post-glacial boundary was about 8300B. C., the Boreal/Atlantic boundary at about 5000 B. C., and the Sub-boreal/Sub-atlantic boundary at about 3000 B. C. The other zone datings conform with this sequence. This evidence goes some way to establish the synchroneity of the major zones on both sides of the North Sea and indicates the suitability of the method for resolving this type of quater­nary historical problem.

Radiocarbon dating and post-glacial vegetational history: Scaleby Moss.

A proportional gas-counter of about 21. volume, within a castle of 5 tons of zinc, enclosing in turn an anticoincidence shield, has been used to make radiocarbon assays of highly purified carbon dioxide at a pressure of 2 atm. A net contemporary count-rate of 27 counts per minute has been secured with a constant background rate of 25. The chief features of this apparatus and of the combustion technique are briefly indicated. The radiocarbon assays have been employed to provide age determinations from a series of peat samples secured from a dry peat-bog at Scaleby Moss, Cumberland. The whole peat sequence has been pollen-analyzed and each radiocarbon sample is very exactly referred to a major zone boundary in a zone sequence which extends from the Late-glacial period continuously through almost the whole of the Post-glacial period. The dates obtained have been based upon an estimated half-life for radiocarbon of 5568 ± 30 years. The errors of determination are of the order (1 S. D.) of ± 150 years. The results are satisfactorily self-consistent, contiguous samples have closely similar dates, and the whole series of dates follows the stratigraphical sequence. It is shown that at Scaleby Moss the Late-glacial/Post-glacial boundary was about 8300 B. C.; The Boreal/Atlantic boundary at about 5000 B. C., and the Sub-boreal/Sub-atlantic boundary at about 3000 B. C. The other zone datings conform with this sequence. This evidence goes some way to establish the synchroneity of the major zones on both sides of the North Sea and indicates the suitability of the method for resolving this type of Quaternary historical problem.

The Natural Radioactivity ofIn115

An investigation of the possible natural radioactivities of the ${\mathrm{Cd}}^{113}$-${\mathrm{In}}^{113}$ and ${\mathrm{In}}^{115}$-${\mathrm{Sn}}^{115}$ pairs of adjacent isobars gave evidence for a natural beta-radioactivity of indium. Four samples of indium differing in geographical origin were subjected to radiochemical purification and then examined in a screen wall counter arranged with anticoincidence counters and steel shielding. Absorption measurements on these samples gave reproducible results indicative of an energetic natural beta-radiation. The energy was measured by comparing the absorption characteristics of the indium radiation with those of ${\mathrm{Tl}}^{204}$ and ${\mathrm{Co}}^{60}$, whose beta-energies have been measured spectroscopically. Measurement of isotopically enriched samples of indium gave conclusive evidence that the origin of the beta-radiation is the ${\mathrm{In}}^{115}$ isotope. Limits were placed on possible accompanying gamma-radiation. The results indicate that ${\mathrm{In}}^{115}$ emits a natural beta-radiation of 0.63\ifmmode\pm\else\textpm\fi{}0.03-Mev energy with a half-life of 6\ifmmode\pm\else\textpm\fi{}2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ years.