Neutron penetrating power

The nuclear reactor also must be shielded against the emission of radioactive material to the external environment. Suitable radiation controls include both thermal and biological shielding systems. Radiation from alpha particles (a rays) and beta particles ((3 rays) has little penetrating power, but gamma rays have deep penetration properties. Neutron radiation is, however, the primary area of risk. Typically, extremely thick concrete walls are used as a neutron absorber, but lead-lined concrete and special concretes are also used. 

Neutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atom nucleus). A neutron is hundreds of times larger than an electron, but one quarter the size of an alpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but they are also produced from the decay of radioactive elements. Because of its size and lack of charge, the neutron is fairly difficult to stop, and has a relatively high penetrating power. 

The fact that there were three basic decay processes (and their names) was discovered by Rutherford. He showed that all three processes occur in a sample of decaying natural uranium (and its daughters). The emitted radiations were designated a, (3, and y to denote the penetrating power of the different radiation types. Further research has shown that in a decay, a heavy nucleus spontaneously emits a 4He nucleus (an a particle). The emitted a particles are monoenergetic, and, as a result of the decay, the parent nucleus loses two protons and two neutrons and is transformed into a new nuclide. All nuclei with Z > 83 are unstable with respect to this decay mode. 

Fig. 5.16. Penetrating powers of ionizing radiations a-particles can be stopped by paper, p-particles can be stopped by aluminum, y-radiation is weakened by lead, but is never greatly totally blocked neutrons will pass through lead, but will be stopped by thick wax or concrete.

As with X-rays, neutrons have found application in surface science in the last few years only (for recent reviews see refs. 175—177). Because of its zero charge, a neutron interacts weakly with matter and therefore has a high penetration power, even at low energies (generally % 1 mm). This feature essentially restricts the substrates on which surface effects can be detected to those which have a high surface area and which... 

Both small angle X-ray (SAXS) cind neutron scattering (SANS) are established techniques and their experimental application is similar. However, limitations on sample size, thickness and containment are much more restricted with X-rays because of absorption of radiation. One problem which can arise with neutrons is the subtraction of the flat incoherent contribution which can be quite large in the case of hydrogenous materials. This disadvantage can be partially offset by the possibility of using isotopic substitution. SANS is particularly powerful because the penetrating power of neutrons makes it possible to study material microstructure in the wet state. Instrumentally, both SAXS and SANS require a source of radiation, collimation system, sample containment and a detection system. 

Earth s atmosphere is constantly being bombarded by cosmic rays of extremely high penetrating power. These rays, which originate in outer space, consist of electrons, neutrons, and atomic nuclei. One of the important reactions between the atmosphere and cosmic rays is the capture of neutrons by atmospheric nitrogen (nitrogen-14 isotope) to produce the radioactive carbon-14 isotope and hydrogen. The unstable carbon atoms eventually form C02, which mixes with the ordinary carbon dioxide ( C02) in the air. As the carbon-14 isotope decays, it emits f3 particles (electrons). The rate of decay (as measured by the number of electrons emitted per second) obeys first-order kinetics. It is customary in the study of radioactive decay to write the rate law as... 

Fig. 2.2 A comparison of the penetrating powers of a-particfes, P-particles, y-radiation and neutrons. Neutrons are especially penetrating and their use in a nuclear reactor calls for concrete-wall shields of >2m in thickness.

Most biological matrices contain considerable amounts of sodium and chlorine which, after activation, emit gamma rays with great penetrating power 1368.6 and 2754.0 keV ( Na) or 1642.4 and 2167.5 keV ( CI). Thus, 1 mL of serum contains approximately 3.25 mg of sodium and 3.95 mg of chlorine. After irradiation for 5 h in a neutron flux of 4 10 n-cm -s the first gives rise to approximately 1 mCi or 37 MBq of Na (half-life 14.959 h), the second to approximately 0.75 mCi or 28 MBq of CI (half-life 37.21 min). Obviously. Na forms the most serious problem. 

The hypothesis Chadwick proposed adopting should come as no surprise If we suppose that the radiation is not a [gamma] radiation, but consists of particles of mass very nearly equal to that of the proton, all the difficulties connected with the collisions disappear, both with regard to their frequency and to the energy transfer to different masses. In order to explain the great penetrating power of the radiation we must further assume that the particle has no net charge. We may suppose it [to be] the neutron discussed by Rutherford in his Bakerian Lecture of 1920. ... 

Applying this correction to the constant A improves the consistency of the data. This correction is applied only to the volume effect since the penetrating power of these neutrons is too great to cause surface absorption. The correction to A changes /t slightly since fi is calculated in terms of A. The corrected fi is listed with A and the uncorrected k, c, and /t in Table II. 

Actually the rate of heat production increases somewhat toward the surface of the rod, because of decreased neutron penetration into the interior, and this increases the power output for a given temperature drop. For a rod of 1.7 cm radius this effect increases the output by a factor of 1.1. 

There are several ways to estimate the water layer necessary for such a reduction. The quickest safe way that one can easily think of is to assume that the collisions do not deviate the neutrons from their original direction. Under this assumption the neutrons evidently have a greater penetrating power than they have in reality. 

Tumor cells are killed or damaged by exposure to the alpha particles. Healthy tissue farther away from the tumor is unaffected because of the short-range penetrating power of alpha particles. Thus, neutron-capture therapy has the promise to be a silver bullet that specifically targets unhealthy cells for exposure to radiation. 

Neutrons - neutrons are emitted during nuclear fission and have very great penetrating powers. They can cause intense ionization. Bremsstrahlung - electromagnetic radiations produced by the slowing down of a p particle. They can have considerable penetrating powers. 

Alpha Particles Alpha particles have the same structure as the nuclei of helium atoms two protons and two neutrons. Relative to other forms of ionizing radiation, alpha particles are large. They have little penetrating power. A piece of paper or the outer layer of skin can stop them. 

The atmosphere of Earth is constantly being bombarded by cosmic rays of extremely high penetrating power. These rays, which originate in outer space, consist of electrons, neutrons, and atomic nuclei. One of the important reactions between the atmosphere and cosmic rays is the capture of neutrons by atmospheric nitrogen (nitrogen-14 isotope) to produce the radioactive carbon-14 isotope and hydrogen ... 

When combined with complementary measurements, such as locally resolved current measurements, the information value of neutron radiographic measurements can be further enriched. Taking advantage of the penetration power of neutrons, a dedicated cell design including a multi-layer printed circuit board with an embedded matrix of shunt resistors was used to measure the local current densities and the liquid water distributions concurrently [24]. With the combination of these methods, it is possible to correlate the local water content with the local electric performance of the cell. Figure 18.14 shows the superposition of the... 

For loss of coolant accident, it has been assumed that coolant is unavailable in the upper plenum, core and lower plenum of the reactor. Due to the absence of a heat removal medium, temperatures of the core will start increasing, leading to heating of all core components. The negative void reactivity coefficient will limit the power and thus, the temperature of the core components. The neutronically limited power would reach 200 kW(th). For this case, a system of 12 variable-conductance heat pipes, made of a carbon-carbon composite with a metallic liner, has been provided. These heat pipes penetrate the core. The condenser end of these heat pipes extends beyond the upper plenum and the interface vessels of heat-utilizing systems to the atmosphere. At the condenser end, these heat pipes have radiator fins to dissipate heat to the atmosphere. In case of a postulated accident due to loss of load or loss of coolant, core temperature will start increasing. As long as the temperature of the core is within... 

Alpha particles, composed of 2 protons and 2 neutrons and given the symbol He. Alpha particles have a high ionizing power but a low penetrating power. 

Beta particles, electrons emitted from atomic nuclei when a neutron changes into a proton. Beta particles have the symbol i e and have intermediate ionizing power and intermediate penetrating power. 

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