Anti-coincidence counter

Beta counting is not isotope specific. It requires relatively simple equipment. A gas-flow anti-coincidence counter with 10 cm lead shielding is frequently used, giving background values of 0.15-0.20 cpm for a source of 25 mm diameter (Ris0 National Laboratory, Ros-kilde, Denmark). It is the preferred method for Th, can be used with non-destructive techniques (Sections 13.6.2.2 and 13.6.3.2), and can be used on-board ship. 

La, the 89Sr + "Sr, and the "Y samples are determined in low background (0.2 c.p.m.) anti-coincidence 0-counters having a sensitivity for measurement of 0-activity of < 1 d.p.m. The disintegration rates are extrapolated back from time of measurement to midtime of the rainfall collection period. 

In Part 2A, the student will calibrate a gas-flow, end-window, anti-coincidence proportional counter for beta-particle counting efficiency as function of energy with certified standard solutions, and perform quality assurance (QA) counting tests. 

Figure 2A.1 Cross-sectional view of a low-level anti-coincidence beta-particle counter A. Sample on a planchet. B. Thin window detector. C. Guard detector. Lead shielding surrounds the entire detector system. Typical background count rates are about 1 count per minute for beta particles and 0.1 count per minute for alpha particles. A sample mounted on a planchet (A) is placed below the thin window. When the guard detector (C) is triggered by an extraneous radiation that penetrates the lead shield, the sample detector (B) is inactivated. Immediately following, the detector (B) responds to beta particles from the sample. For low-activity samples, the probability is low that a particle from the sample registers a pulse at the same time that the counter is inactivated.

Marshall, Marshall, and Nedzeli and de Carvalho 2 at the lower energies used very thin samples. This meant that the transmission of the samples was close to unity. To avoid the difficulties of exactly measuring almost equal counting rates, the measurements at the lower energies were made by an anti-coincidence technique. Specifically the counter behind the sample was in anti-coincidence with those in front of the sample and the number of protons removed from the beam was measured. They also varied the solid angle of the detector behind the sample the angles subtended varied from 2 to 15 . 

Similar to the PIP, the Hamiltonian [Eq. (52a)] of a periodic pulse shows an infinite number of effective RF fields with both x and y components of the scaling factors X a and the phases 0na. The periodic pulse, however, acquires a different symmetry as that of the PIP. From Eq. (52c) and = ana, it follows that the scaling factor Xm, is symmetric in respect to the sideband number n, while the phase 6na is anti-symmetric according to Eq. (51c). These symmetries seem to be a coincidence arising from the mathematical derivations. As a matter of fact, they are the intrinsic natures of the periodic pulse. Considering the term f x i)Ix for instance, any Iy component created by the rotating field denoted by a> must be compensated at any time t by its counter-component oj n in order to reserve the amplitude modulated RF field. 

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