Nuclear Radiation Detectors

instruments that register alpha and beta particles, X rays, gamma rays, neutrons, and protons.

Nuclear radiation detectors serve to determine the composition and measure the intensity of radiation, to measure the energy spectra of particles, to study the processes of interaction between fast particles and atomic nuclei, and to study the decay processes of unstable particles. Detectors that make it possible to imprint the trajectories of individual particles—the Wilson cloud chamber, a variety of the Wilson chamber known as the diffusion chamber, the bubble chamber, the spark chamber, and nuclear photographic emulsions—are especially useful in the study of the decay processes of unstable particles, the most complex group of problems. The operation of all nuclear radiation detectors is based on the ionization or excitation by charged particles of the atoms of the substance that fills the effective volume of the detector. In the case of y-quanta and neutrons, ionization and excitation are accomplished by secondary charged particles that arise as a result of the interaction between gamma quanta or neutrons and the working medium of the detector. Thus, the passage of all nuclear particles through the medium is accompanied by the formation of free electrons and ions, the appearance of flashes of light (scintillations), and chemical and thermal effects. As a result, radiation can be registered by the appearance of electrical signals (current or potential pulses) at the output of the detector, by the darkening of a photoemulsion, or by other means. The electrical signals are usually small and require amplification. The current intensity at the output, the average pulse recurrence frequency, and the degree of darkening of the photoemulsion are measures of the flux intensity of the nuclear radiation.

An important characteristic of nuclear radiation detectors that register individual particles is their efficiency—the probability of the registration of a particle upon entry into the effective volume of the detector. Efficiency is a function of the design of the detector and the properties of the working medium. For charged particles (with the exception of very slow particles) the efficiency is close to unity; the registration efficiency for neutrons and y-quanta is usually less than unity and depends on their energy. It is often necessary that a nuclear radiation detector be sensitive only to particles of a single type (for example, a neutron detector should not register gamma quanta).

The ionization chamber, a gas-filled charged electric capacitor housed in a hermetic chamber, is the simplest of the nuclear radiation detectors. If a charged particle enters the chamber, a current induced by ionization of the atoms of the gas arises in the electric circuit that is connected to the electrodes of the chamber; the current intensity is a measure of the flux intensity of the particles. The chambers are also used in a mode that records the potential pulse effected by an individual particle; the magnitude of the pulse is proportional to the energy lost by the particle in the gas within the chamber. Ionization chambers register all types of nuclear radiation, and their design and the composition of the gas depend on the type of radiation to be registered.

As the potential difference between the electrodes of the chamber increases, the electrons that appear in the effective volume of the chamber acquire energy sufficient for the secondary ionization of the neutral gas molecules while moving toward the electrode. The potential pulse at the output increases because of this phenomenon and can be registered more easily. The operation of a proportional counter, which is used to measure the flux intensity and energy of particles and quanta, is based on this principle.

In a Geiger-Müller counter the electric field intensity between the electrodes is still greater, which leads to an increase in the ionization current from secondary ionization. The pulse amplitude at the output ceases to be proportional to the energy of the primary particle; the amplitude becomes extremely large, thus facilitating registration of the pulses. Because of their simplicity of design, Geiger-Müller counters are widely used for registering alpha and beta particles and gamma quanta.

The operation of a scintillation counter is based on the phenomenon of fluorescence, which occurs through the interaction of nuclear particles and scintillators—special liquids, plastics, crystals, and noble gases. A light flash is registered by a photoelectric multiplier, which converts it into an electrical impulse. Scintillation detectors are highly efficient for y-quanta and respond quickly. The amplitude of the output signal is proportional to the energy transferred to the scintillator by the particle; this fact makes it possible to use these detectors to measure the energy of nuclear particles. The high efficiency of scintillation detectors is a result of the fact that, unlike ionization chambers, proportional counters, and Geiger-Müller counters, the working medium of the detector is dense and its absorptivity is approximately 10³ times greater than the absorptivity of a gas at a pressure of ≈ atmosphere.

A crystal counter is also highly efficient. Its operation is similar to the operation of an ionization chamber. Where a charged particle forms free electrons and ions in an ionization chamber, electron-hole pairs appear in a crystal dielectric counter (using diamond, zinc sulfide). Crystal counters are used comparatively infrequently.

The use of semiconductor crystals (usually of silicon or germanium with a lithium admixture) as the working medium makes it possible to obtain, in addition to high efficiency, excellent energy resolution that exceeds the resolving power of scintillation detectors and that is comparable to the resolution attainable in less luminous magnetic spectrometers. Therefore, semiconductor detectors are used widely for precision measurements of the nuclear radiation energy spectrum. Some types of semiconductor detectors must be cooled to temperatures close to that of liquid nitrogen.

The Cherenkov counter, based on the registration of Cherenkov-Vavilov radiation, is used to measure the energy of very fast particles. Dielectric detectors are sometimes used to register fast heavy ions, such as nuclear fission fragments.

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