Measurement anticoincidence

Notice that the rate at which the pulse rises (risetime) is determined by the decay time T. In certain measurements, e.g., coincidence-anticoincidence measurements (Chap. 10), the timing characteristics of the pulse are extremely important. 

For a successful coincidence or anticoincidence measurement, the detector signals should not be delayed by any factors other than the time of arrival of the particles at the detector. If it is known that it takes longer to generate the signal... 

Anticoincidence measurement of protons removed from beam. Gas counters and scintillation counters... 

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%. 

The UW detector uses Sharp low beta anticoincidence electronics and scalers (low beta I) with voltage sensitive preamplifiers (Sharp is a division of Beckman Instrument Co.). The electronic window selected by a pulse generator is 0.75-100 mv. input for the anticoincidence channel and 0.5-oo mv. for the guard channel. The discriminators were set up for use only in natural radiocarbon measurements. 

Ge(Li) Detector Characteristics. Resolution measurements for the 18-cm.8 Ge(Li) detector were made with the anticoincidence shield in the inoperative mode, with a normal operating bias of 1700 volts, and with a preamplifier designed in our Laboratory (3, 4), and operated in conjunction with a Tennelec TC-200 linear amplifier. Resolution at 1.33 M.e.v. was 2.62 k.e.v., FWHM (Figure 4). The electronic pulser resolution for the amplifier system at a slightly lower energy was 1.86 k.e.v., the total capacitance of the detector was 28 pF, the noise slope was 0.035 k.e.v./pF, and the leakage current at 1700 volts was 0.5 X 10"9 amp. 

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)... 

An application of anticoincidence circuits is the anti-Compton spectrometer. The Compton continuum in y spectra can be reduced relative to the photopeaks by placing the Ge detector inside a second detector, usually a scintillation detector, connected in anticoincidence, so that only pulses in the central detector that are not coincident with those in the outer detector are recorded. Anti-Compton spectrometers are very useful for measurement of y rays of very high energy. 

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. 

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. 

From 3 to 12 Mev, measurements of the total cross sections in a large number of elements have been made by Nereson and Darden using the neutrons emitted by a fast neutron reactor as a source, and a special energy-selective detector. In that instrument, protons projected in the forward direction from a polythene foil were measured by an ionisation chamber connected in coincidence with a proportional counter located between it and the foil, and in anticoincidence with another counter beyond it. The energy of the protons was measured by the size of the pulses they produced in the chamber the energy resolution was about 10%. 

Counter combinations with anticoincidence circuit (measuring counter surrounded by a ring of protective counters which lower the zero effect). 

Large-area counters with anticoincidence circuit and appropriate lead screening, combined with a suitable measuring and evaluation unit, are frequently selected as detectors in water analysis. 

Methane flow counters have an efficiency of around 50 %, on account of the positive geometry of the measuring unit. The zero effect in flow counters is in the region of 20 pulses/min., but this counter equipment can nevertheless compete successfully with an anticoincidence circuit as a result of its high efficiency. 

On the other hand, the effect of ambient radiation on the background of the measurement set-up must be reduced. This is effected by way of anticoincidence systems to eliminate the effect of cosmic radiation, lead shields to attenuate the ambient gamma radiation, and extensive use of electrolytic copper with a particularly low radioactivity level as detector material. Nowadays, beta background rates considerably below 10 pulses per minute are achieved with modern low-level systems employing 200 mm dishes. 

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