Beta particle detection from

This metal chamber serves to shield the plates from outside electric fields and to contain the air or other gas. Gamma rays have very little trouble in penetrating the metal walls of the chamber. Beta particles and alpha particles, however, are stopped by the metal wall. For alpha and beta particles to be detected, some means must be provided for a thin wall or "window." This window must be thin enough for the alpha and beta particles to penetrate. However, a window of almost any thickness will prevent an alpha particle from entering the chamber. 

When using an ionization chamber for detecting neutrons, beta particles can be prevented from entering the chamber by walls thick enough to shield out all of the beta particles. Gamma rays cannot be shielded from the detector therefore, they always contribute to the total current read by the ammeter. This effect is not desired because the detector responds not only to neutrons, but also to gamma rays. Several ways are available to minimize this problem. 

In 1899 he identified two forms of radioactivity, which he called alpha and beta particles. As we saw earlier, he deduced that alpha particles are helium nuclei. Beta particles are electrons - but, strangely, they come from the atomic nucleus, which is supposed to be composed only of protons and neutrons. Before the discovery of the neutron this led Rutherford and others to believe that the nucleus contained some protons intimately bound to electrons, which neutralized their charge. This idea became redundant when Chadwick first detected the neutron in 1932 but in fact it contains a deeper truth, because beta-particle emission is caused by the transmutation ( decay ) of a neutron into a proton and an electron. 

Table 32-1 lists the most important (from a chemist s viewpoint) types of radiation from radioactive decay. Four of these types — alpha particles, beta particles, gamma-ray photons, and X-ray photons —can be detected and recorded by the detector systems described in Section 12B-4. Most radittchcniical methods are based on counting the electronic signals produced when these decay particles or photons strike a radiation detector. 

Radiation from radioactive sources can be detected and measured in essentially Ihe same way as X-radialioii (Sections l2B-4and 12B-S). Gas-filled chambers, scintillation counters, and semiconductor detectors are all sensitive to alpha and beta particles and to gamma rays because absorption of these particles produces ionization or photoelectrons, which can in turn produce thousands of ion pairs. A detectable electrical pulse is thus produced for each particle reaching the transducer. 

A gas flow Geiger tube was used to count the and radiation from the films. The radiation from and H can be distinguished because the energies of the beta-particles emitted in each case, and thus their penetrating powers, are different. C radiation was detected through a thin Mylar window on the Geiger tube. H radiation does not penetrate this window. The window was removed to count the H and C radiation combined. The net counting rate for radiation was ob-... 

In a liquid scintillation (LS) system, the sample is mixed with a cocktail that consists of an organic scintillator dissolved in an organic solvent. The cocktail and the usual aqueous sample form an emulsion. The radiation emitted by the intimately mixed radionuclide deposits its energy in the solvent, which transfers it to the scintillator. The scintillations are then detected by the PMT. The LS counter is useful for detecting alpha particles and low-energy beta particles from samples that... 

The G-M counter is a simple and relatively inexpensive gas-filled tube with a count-rate meter an amplifier may also be present. The G-M detector counts alpha particles, beta particles, and gamma rays with a very thin window, counts beta particles and gamma rays with a thicker window, and counts gamma rays only with a thick shield. Alpha and beta particles interact in the gas gamma rays interact mostly in the walls, from which electrons enter the gas. The intrinsic efficiency for counting gamma rays relative to beta particles depends on the amount and type of solids surrounding the detection gas. 

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