Ra-Be neutron sources

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... 

The SUR-IOO reactor is a solid homogenous reactor with 20% enrichment uranium fuel, moderated by polyethylene and using graphite as reflector. The core has the form of a cylinder, with 24 cm in diameter and 26 cm high, and is cooled by natural circulation of air. It is made up of fuel elements in the form of polyethylene discs of 24 cm diameter and stacked to a total height of about 26 cm divided into two symmetrical blocks the upper block is fixed and the lower one is movable. Two cadmium plates outside the core — in the reflector region — are used as control rods. The reactor also has a Ra-Be neutron source. 

The calibration was accomplished by placing a Ra-Be neutron source next to the counting pig and then manually triggering the MCA while simultaneously starting a stop watch. After a known elapsed time interval, the pulse input to the MCA was blocked by switching the input selector switch on the MFC from the amplifier to the GM position. After another known time interval the pulse input to the MCA was returned. 

Activation analysis is the other field of radiochemical analysis that has become of major importance, particularly neutron activation analysis. In this method nuclear transformations are carried out by irradiation with neutrons. The nature and the intensity of the radiation emitted by the radionuclides formed are characteristic, respectively, of the nature and concentrations of the atoms irradiated. Activation analysis is one of the most sensitive methods, an important tool for the analysis of high-purity materials, and lends itself to automation. The technique was devised by Hevesy, who with Levi in 1936 determined dysprosium in yttrium by measuring the radiation of dysprosium after irradiation with neutrons from a Po-Be neutron source. At the time the nature of the radiation was characterized by half-life, and the only available neutron sources were Po-Be and Ra-Be, which were of low efficiency. Hevesy s paper was not followed up for many years. The importance of activation analysis increased dramatically after the emergence of accelerators and reactors in which almost all elements could be activated. Hevesy received the 1943 Nobel prize in chemistry for work on the use of isotopes as tracers in the study of chemical processes . 

To avoid predetonation one must make sure that there is only a small probability of a neutron appearing while the pieces of the bomb are being put together. On the other hand, when the pieces reach their best position one wants to be very sure that a neutron starts the reaction before the pieces have a chance to separate or break. It may be possible to make the projectile seat and stay in the desired position. Failing this, or in any event as extra insurance, another possibility is to provide a strong neutron source which becomes active as soon as the pieces come into position. For example one might use a Ra+Be source in which the Ra is on one piece and the Be on the other so neutrons are only produced when the pieces are close to the proper relative position. 

A Ra-Be source provides the neutrons for Fermi s studies. These studies lead to the construction of a nuclear reactor. 

The uranium decay series provides the most important isotopes of elements radium, radon, and polonium, which can be isolated in the processing of uranium minerals. Each ton of uranium is associated with 0.340 g of Ra. Freshly isolated Ra reaches radioactive equilibrium with its decay products to Pb in about two weeks (see Fig. 1.2). Many of these products emit energetic y-rays, which resulted in the use of Ra as a y-source in medical treatment of cancer (radiation therapy). However, the medical importance of radium has diminished greatly since the introduction of other radiation sources, and presently the largest use of radium is as small neutron sources (see Table 12.2). 

In order to obtain flows of charged particles of a definite energy, accelerators (for instance, a cyclotron) are used. The irradiation with neutrons is carried out in reactors, where powerful flows of neutrons are formed due to the reaction ( ,/). At laboratory scale, small sources of neutrons are used, e.g., Ra-Be (reaction Be(a,n) C, 10 neutrons/s, but under a considerable y- background ) or Po-Be (not only less danger of y-radiation, but also less neutron yield and low half-life of Pb [140 days]). Neutron energies for these sources are in the range of 1-8 MeV. To get thermal neutrons, these sources are put inside a moderator, for instance, water or paraffin hydrocarbon. 

In the dawn of the history of nuclear science, the neutron flux density (/) of Ra-Be source was only 10 -10 n cm s The total activity produced by such neutron sources via (n,y) reaction was very low and so was the specific activity of the radionuclides. The Szilard-Chalmers process, however, could dramatically increase the specific activity the improvement could reach orders of magnitude. In the measurement of P radioactivity, which was a frequent task in early days of nuclear science, samples with low specific activity brought sometimes troublesome problems of self-absorption corrections. By the introduction of the Szilard-Chalmers process, however, this difficulty could be avoided, because the measurement could be performed within small statistical errors using a sample with high specific activity. Therefore, the Szilard-Chalmers process became one of the useful means of preparation of radioisotopes for measurement, as Szilard and Chalmers (1934b) recognized the importance of this technique in their early work. 

At the beginning of the 20th century, radium was used to estabhsh the standard prototype of the curie. Actually, one curie is exactly equal to the radioactivity of a source that has the same radioactivity as 1 g of the radionuchde radium-226 in secular equilibrium with its derivative, radon-222 (or emanation). In spite of the new mandatory SI unit of radioactivity, the bec-querel, symbol Bq, the curie, Ci, is sometimes still in use (1 Ci = 37 GBq). Twenty-five isotopes are now known radium-226, the most common isotope, has a half-life of 1620 years. This isotope is purged from radium and sealed in minute tubes, which are used in the treatment of cancer and other diseases. Radium is used in the production of self-luminous paints, neutron sources, and in medicine for the treatment of disease. Some of the more recently discovered radioisotopes, such as Co or Cs, are now being used in place of radium. Some of these sources are much more powerful, and others are safer to use. Inhalation, injection, or body exposure to radium can cause cancer and other bodily disorders. The maximum permissible burden in the total body for Ra is 7.4 kBq. 

UWNR uses a Pu-Be source for a "bugging" and calibration source, while the reactor startup source is a special Ra-Be source. The special part is that it has been irradiated to give a higher output of particles, and, thus, neutrons. It actually contains 100 milligrams (100 millicuries) of Ra, but it emits the same number of neutrons as an linirradiated Ra source of 2 curies ( 2 x lO n/sec). ... 

Outline the steps involved in measurement of absolute neutron flux by means of a standard pile. How would you measure the neutron yield of a Ra-Be source ... 

The determination of isotopic abundance by means of an n,p) reaction induced by thermal neutrons has been described by Coon (16). Variations in the He content of helium gas depending on its source had been recorded and Coon was able to confirm these with gas obtained from wells and from air by means of the reaction He (n,p)H whose cross section for thermal neutrons is approximately 5,000 bams. A search for Si in natural silicon was conducted by Turkevich and Tomkins (WB). Theory had indicated that Si might be a beta-stable isotope and occur in natural silicon in small, undetected amounts. Neutron-irradiated quartz was examined radiochemically for 25-day half-life P , the daughter of Si formed by (ra,y) reactions on the sought isotope. However, only P , probably formed from impurities, was detected and assuming a cross section for the Si (n,y) Si reaction of 0.05 bams an upper limit of 4 x 10 % results for the abundance of Si in natural silicon. Subsequent work has shown Si to be an approximately 300-yr half-life beta emitter. 

Predetonation would reduce the bomb s efficiency, Berber repeated so also might postdetonation. When the pieces reach their best position we want to be very sure that a neutron starts the reaction before the pieces have a chance to separate and break. So there might be a third basic component to their atomic bomb besides nuclear core and confining tamper an initiator— Ra -h Be source or, better, a Po -t- Be source, with the radium or polonium attached perhaps to one piece of the core and the beryllium to the other, to smash together and spray neutrons when the parts mated to start the chain reaction. 

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