Anticoincidence counting

A linear amplifier system designed by Goulding (10) includes the necessary circuits for anticoincidence counting. The system is contained in a 7-inch high standard-rack mounting chassis. Pulse-shaping from... 

For certain measurements, like coincidence-anticoincidence counting or experiments involving accelerators, the time resolution of the signal is also important, in addition to energy resolution. For timing purposes, it is essential to have pulses with constant risetime. 

Some other scintillation materials, such as cesium iodide and bismuth ger-manate, have characteristics that are less favorable than Nal(Tl) for general use, but recommend them for some special measurements. For example, Csl and Nal(Tl) can be combined for coincidence or anticoincidence counting by distinguishing between output from the two detectors by their pulse shapes. 

Liquid scintillation counting has been used frequently for the measurement of environmental technetium. The specimens to be analyzed are treated by chemical procedures to obtain a technetium-bearing sample solution, which is mixed with a cocktail for scintillation counting. A low background scintillation counter with an anticoincidence system can be used for high precision measurements at a detection limit of 1-25 mBq. 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

A drive mechanism raises the annulus for inserting the sample. Most samples are accommodated in a position just above the midline of the anticoincidence shield. The Ge(Li) detector is just above the sample a raise-lower mechanism provides approximately a 10-cm. vertical range in its position. The counting of small planchets does not require separation of the annuli a long plastic tongue may be used to insert the sample through a small door in the shield. 

Left without anticoincidence shielding. Right with shielding. The presence ing. The sample was counted in 200 ml. of water in each... 

Anticoincidence, Coincidence Recording. An inherent problem in an anticoincidence-shielded spectrometer is that a decay scheme having gamma-rays in coincidence will appear to produce the same interaction between the Ge(Li) detector and anticoincidence shield as that of a Compton event. If only the events from the Ge(Li) detector are counted in the absence of a signal from the shield, those nuclides that decay... 

Nal(Tl) crystals. The sample to be counted is mounted between Mylar sheets on a 1/4-inch thick X 5-inch diameter plastic ring. This system is presently used with a four quadrant, multiplexed, 4096-channel, multidimensional analyzer. The multidimensional y spectrum in coincidence with the proportional counter events is recorded in one quadrant, and the anticoincident multidimensional y spectrum is recorded in another quadrant thus, no data are lost. The various background y spectra obtained with this detector assembly are compared in Figure 12. The y-y coinci-... 

Figures 4 and 5 also indicate that a decay period of approximately 25-30 days reduces the intermediate-lived radionuclides such as 24Na, 140La, and 82Br to insignificant activity levels without seriously affecting the ability to measure the remaining isotopes of interest. The second count is then conducted for 100-1000 min on a Ge(Li) or anticoincidence shielded Ge(Li) spectrometer (28) and provides concentrations for Ag, Ba, Ce, Co, Cr, Cs, Eu, Fe, Hf, Hg, Ni, Rb, Sb, Sc, Se, Sr, Ta, Tb, Th, and Zn.

Natural Radioactivity Measurement. The naturally occurring radionuclides U(226Ra), Th(232Th), and K(40K) can also be determined on 100-g samples by direct counting of unirradiated samples using anticoincidence shielded multidimensional y-ray spectrometers (29, 30). These spectrometers use large (12 in. diameter X 8 in thick) principal Nal(Tl)... 

A number of special techniques have evolved to increase the detection sensitivity in y-ray counting. One of the most important is the suppression of the Compton scattering events in the y-ray spectrum by the use of anticoincidence annulus around the central y-ray detector. The idea behind a Compton suppression spectrometer is that most events in which the incident photon undergoes one or more Compton scattering events in the central detector will result in partial energy deposition in the detector with a low-energy photon escaping the detector. 

A gas proportional counter with an anticoincidence shielding counter has also been utilized. Ethane gas is commonly used as a counting gas. A water sample is reduced to hydrogen gas using Mg or Zn powder in a furnace. Hydrogen gas thus produced is added to dead ethylene or acetylene gas with a Pd catalyst to form ethane-counting gas. 

If samples of very low activity are to be measured, the contribution of the background to the counting rate and hence the error of the measurement are relatively high. The influence of the background can be reduced by intensiflcation of the detector shielding and by coincidence or anticoincidence circuits. 

In the more than 30 years that have followed, radiocarbon studies have evolved through two generations of instrumental methods. Libby employed solid carbon counting (combined with an application of the anticoincidence principle to reduce background count rate) to establish the fundamental validity of the method (6). This goal was achieved in December 1949 with the publication of the famous Curve of Knowns, which demonstrated that the residual content of a series of samples was directly related to their age (7). 

Figure 5.26 A low-background alpha-beta counting system utilizing two counters and anticoincidence. The anticoincidence output gates the scaler to count only pulses from the gas-flow counter.

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