Fission Neutron Source

The current availability of small portable 14 MeV neutron generators and the future availability of high intensity 252Cf spontaneous fission neutron sources will certainly result in the wide spread use of activation techniques for non-destructive "on-stream" product analysis in industry. The cost of the required instrumentation for many types of activation analysis is not excessive, as compared to the cost of other modem analytical instrumentation. The simple off-on operation of the new sealed-tube neutron generators and minimal maintenance associated with the use of an isotopic Z5ZCf neutron source will permit operation of the analytical facility with technician-level personnel. The versatility of the activation technique justifies its inclusion among the other major analytical techniques employed in any modem analytical facility. 

Californium (Cf) 98 2.65 y Activation/transmutation in fission Neutron source and fis-... 

The procedure for neutron measurements involved placing the detector near a neutron source storage drum. Measurements were made with a 3 Ci AmBe neutron source and a 3 mCi Cf spontaneous fission neutron source housed in 55-gallon drums. The drum contained a thick neutron moderator/absorber shield, and, thus, it was assumed that thermal neutrons predominated in the energy distribution outside the drum. The detector and PMT were shielded from fission gamma radiation by 5-cm thick lead bricks, and an additional 3-cm thick, high-density polyethylene moderator was interposed between the bricks and the source drum. Pulse-height spectra were accumulated over 1.5 x 10 s for each sample. 

Test results show that moderated fission-neutron sources of strength about 3 x 10 n/s can be detected at a distance out to 70 m in a counting time of 1000 s. The best angular resolution of the detector is obtained at distances of 30 m or less. As the separation distance between the source and detector increases, the contribution of scattered neutrons to the measured signal increases with a resultant decrease in the ability to detect the direction to a distant source. 

Uses of Plutonium. The fissile isotope Pu had its first use in fission weapons, beginning with the Trinity test at Alamogordo, New Mexico, on July 16, 1945, followed soon thereafter by the "Litde Boy" bomb dropped on Nagasaki on August 9, 1945. Its weapons use was extended as triggers for thermonuclear weapons. This isotope is produced in and consumed as fuel in breeder reactors. The short-Hved isotope Tu has been used in radioisotope electrical generators in unmanned space sateUites, lunar and interplanetary spaceships, heart pacemakers, and (as Tu—Be alloy) neutron sources (23). 

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

The detonation of nitrogen iodide by nuclear fission was first reported by Feenberg (Ref 13). Small samples of nitrogen iodide mixed with black uranium oxide were exposed to a 20Qmg Ra-Be neutron source surrounded by 6cm of paraffin. A typical sample contd l/2g uranium... 

Radioisotopes that decay by spontaneous fission with the direct accompanying release of neutrons are usually associated with the natural elements of uranium and thorium and the manmade element plutonium. However, the rate of decay of these elements by fission is so slow that it is only by incorporating them into large nuclear piles or chain reactors that they can be utilized as intense neutron sources. In the US Dept of Energy National Transplutonium Program, small quantities of elements heavier than plutonium are produced for basic research studies and to discover new elements with useful properties. One of these new elements, californium-252 (2S2Cf), is unique in that it emits neutrons in copious quantities over a period of years by spontaneous fission... 

More complex is the production of Pu-238 (used for isotopic heat sources) and californium-252 (used in research as a source of fission neutrons). For Pu-238 there are two... 

In SRP s the primary neutron sources are induced by spontaneous fission of the built-up transuraniinn isotopes " °Pu), as well as beryllium (a,n)... 

In order to control subcriticality of SRP s storage facilities by the stationary method, it is necessary to know the induced fission neutron generation rate per gram of fuel (Qind) and the rate of neutron generation by above-mentioned sources per gram of fuel (Qsp) in the maximum neutron flux points [17], If Qsp and Qmd are known, the system multiplication can be written in the following way ... 

Figure 2 Improvement in Neutron Fluxes (vertical axis) over time (horizontal axis). Note the predominance of fission reactor sources from 1945 to 1980 (dashed line in the middle) and the steady drift toward more intense spallation sources in more recent years (solid line to the right). (Drawn from data in Skold Price, 1986)...

Due to the production of neutrons, spontaneously fissioning nuclides are of practical interest as neutron sources. An example is Cf ( i/2 = 2.64 y), which is used for neutron activation. 

Spontaneously fissioning radionuclides may be applied as neutron sources in those cases in which irradiation in a nuclear reactor is not possible, for example if manganese nodules at the bottom of the sea are to be analysed. For that purpose, Cf is a suitable neutron source. It has a half-life of 2.645 y and decays in 96.9% by emission of a particles and in 3.1% by spontaneous fission. It may be installed together with a shielded y-ray detector in the form of a mobile unit. The neutron production of Cf is 2.34 lO s g. The neutron flux density is only of the order of 10 cm s , but this is sufficient for applications in which high sensitivity is not needed. 

A number of methods have been used to generate sufficiently strong neutron sources, but all current facilities are either fission reactors or spallation sources, so we will consider only these two methods. 

There are several types of neutron sources one can use for NAA, but megawatt nuclear reactors with their intense flux of 10 to 10 neutrons m s from uranium fission, offer the highest available sensitivities for most elements. Neutron energy distribution is quite broad, but for conventional NAA, low-energy neutrons (energies below 0.5 eV) are chosen that represent 90—95 per cent of the neutron flux. Sufficient levels of activation are reached in a few minutes, even if the isotope formed has a long half-life. The procedure imposes that the sample to be treated must be thermally stable. It is enclosed in a tube with a comparator standard of known concentration, before being introduced in a reactor beam port. 

As shown above, neutrons are in particular important for studies of hydride materials. Neutrons can be produced either in nuclear reactors or by pulsed (spallation) neutron sources. In the research nuclear reactors neutrons are produced by fission processes based on U-235 (which is 0.7% in natural uranium, but usually emiched as fuel for reactors). Since the released neutrons from these processes are very energetic, the required chain reaction for continuous production of neutrons requires moderation (to reduce the energy)... 

In nuclear fission, neutron bombardment causes a nucleus to split, releasing neutrons that split other nuclei to produce a chain reaction. A nuclear power plant controls the rate of the chain reaction to produce heat that creates steam, which is used to generate eiectricity. Potential hazards, such as radiation leaks, thermal pollution, and disposal of nuclear waste, remain current concerns. Nuclear fusion holds great promise as a source of clean abundant energy, but it requires extremely high temperatures and is not yet practical. 

Monoenergetic heavy ions necessary for energy calibration can be provided only by accelerators. Fission fragments, which are heavy ions, cover a wide spectrum of energies (Fig. 13.19). The isotope Cf is a very convenient source of fission fragments produced by the spontaneous fission of that isotope. Uranium, plutonium, or thorium fission fragments can only be produced after fission is induced by neutrons therefore, a reactor or some other intense neutron source is needed. 

Isotopic neutron sources are based on (a, ) and (y,n) reactions, and on spontaneous fission ( Cf). They all produce fast neutrons. The (a, ) and (y, n) sources produce the neutrons through the reactions... 

The isotope is the only spontaneous fission (SF) source of neutrons easily available. It provides fission spectrum neutrons with an average of 2.3 MeV. The characteristics of isotopic neutron sources are given in Table 15.4. 

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