Testing whether food has been contaminated by nuclear contamination, which requires the use of professional instrumentation - nuclear contamination detector, or sent to a professional testing center testing or regular monitoring.
Principles and methods of nuclear radiation monitoring
Materials or devices capable of indicating, recording and measuring nuclear radiation. Radiation and nuclear radiation detector within the material interaction and produce some kind of information (such as electricity, light pulses or changes in the structure of the material), after amplification is recorded, analyzed to determine the number of particles, location, energy, momentum, time of flight, speed, mass and other physical quantities. Nuclear radiation detectors are indispensable tools and means for nuclear physics, particle physics research and radiation applications. In accordance with the recording method, nuclear radiation detectors are broadly divided into two categories: counters and trajectory chamber.
Counter ? In the form of electrical impulses to record, analyze some kind of information produced by radiation. Types of counters include gas ionization detectors, multifilament and drift chambers, semiconductor detectors, scintillation counters, and Cherenkov counters.
Professional Nuclear Radiation Detection Instruments
Gas Ionization Detector?
Measures nuclear radiation by collecting the ionizing charge produced by rays in a gas. The main types are ionization chambers, orthogonal counters and Geiger counters. They are similar in structure, generally a cylindrical container with two electrodes, filled with some kind of gas, and a voltage is applied between the electrodes, the difference being a different range of operating voltages. The ionization chamber operates at a lower voltage and directly collects the ion pairs produced by the rays in the gas originally. Its output pulse amplitude is small, with a fast rise time, and can be used for radiation dosimetry and energy spectrum measurements. Positive ratio counter working voltage is higher, can make in the electric field in the high-speed movement of the original ions to produce more ion pairs, in the electrode collected than the original ion pairs are much more than the ion pairs (i.e., the gas amplification effect), so as to get a higher output pulse. The pulse amplitude is proportional to the energy lost by the incident particles and is suitable for energy spectrum measurements. Geiger counter, also known as Geiger-Miller counter or G-M counter, its operating voltage is higher, there are many ionization processes, so the output pulse amplitude is very high, is no longer proportional to the number of ion pairs of the original ionization, can be recorded directly without amplification. It can only measure the number of particles but not the energy, and it takes a long time to complete a pulse count.
Semiconductor detectors ?
Radiation in semiconductors produces carriers (electrons and holes), which are collected under a reverse-biased electric field, and nuclear radiation is measured by the electrical pulse signal generated. Commonly used silicon, germanium as semiconductor materials, there are three main types: ① in the n-type single crystal sprayed with a layer of gold film on the face of the base type; ② in the high resistivity p-type silicon diffusion into a layer of impurities that can provide electrons diffusion junction type; ③ in the p-type germanium (or silicon) the surface of the spraying of a thin layer of lithium metal and drift after the lithium drift type. High-purity germanium detector has a high energy resolution, high detection efficiency of γ radiation, can be saved at room temperature, widely used. Gallium arsenide, cadmium telluride, mercury iodide and other materials are also used.
Scintillation counter ?
Through the charged particles hit on the scintillator, so that the atom (molecule) ionization, excitation, in the de-excitation process of luminescence, after the photoelectric device (such as photomultiplier tube) will be turned into a measurable electrical signal to measure nuclear radiation. Scintillation counter resolution time is short, high efficiency, but also according to the size of the electrical signal to determine the energy of the particle. Scintillator can be divided into three main categories: ① inorganic scintillator, commonly used thallium (Tl) activated sodium iodide NaI (Tl) and cesium iodide CsI (Tl) crystals, they are sensitive to electron, γ radiation, high luminous efficiency, good energy resolution, but the light decay time is longer; bismuth germanate crystals with a high density, high luminous efficiency, and thus the detection of high-energy electron, γ radiation is very effective. Others, such as silver (Ag) activated zinc sulfide ZnS (Ag) is mainly used to detect α particles; glass scintillator can measure α particles, low-energy X radiation, adding the carrier can be measured after the neutron; barium fluoride (BaF2) density, fluorescent components, both for energy measurement, but also for time measurement. Organic scintillators, including plastic, liquid and crystal (such as anthracene, perylene, etc.), the first two are commonly used. Because of their short light decay time (2 to 3 nanoseconds, fast plastic scintillators can be less than 1 nanosecond), commonly used in time measurement. Their detection efficiency of charged particles nearly 100 percent. ③ gas scintillators, including xenon, helium and other noble gases, luminous efficiency is not high, but the light decay time is shorter (<10 nanoseconds).
Cherenkov counter ?
High-speed charged particles moving faster than light in a transparent medium produce Cherenkov radiation, the angle of which is related to the particle velocity, thus providing a detector for measuring the velocity of charged particles. Such detectors are often used in conjunction with photomultiplier tubes; they can be categorized as threshold (recording only particles greater than a certain velocity) and differential (selecting only particles of a certain determined velocity). The counter can be divided into two types: threshold (recording only particles of greater than a certain speed) and differential (selecting only particles of a certain speed). In addition to the above commonly used types of counters, there are also gas proportional scintillation chambers, self-extinguishing streaming light counters, are the recent emergence of gas detectors, the output pulse amplitude is large, the time characteristics of good. Electromagnetic quantum (or cluster emission counters) and hadron quantum can measure the energy of high-energy electrons, γ-radiation or hadrons (see elementary particles), respectively. The crossing radiation counter provides a means for the identification of very high-energy charged particles.
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