Thermal Neutron Reactions

When a target nucleus, zX, is irradiated by thermal neutrons, the reaction induced, known as radiative capture, is represented by  

The capture of the neutron raises the neutron/proton ratio in the compound nucleus formed, favouring P emission. Reaction (18.5) is usually accompanied by y emission from the excited states of z+ Y. 

When a target of mass m is irradiated for time t by thermal neutrons, the activity induced, A, will be given by  

---

Tritium. In natural hydrogen it occurs in amounts of 1 in 1017-1018. It is continuously formed in nuclear reactions induced by cosmic rays, and it is radioactive. It may be made, from lithium, in nuclear reactors by the thermal neutron reaction 6Li( ,a)3H. 

Tritium is formed continuously in the upper atmosphere in nuclear reactions induced by cosmic rays. For example, fast neutrons arising from cosmic-ray reactions can produce tritium by the reaction 14N( , 3H)12C. Tritium is radioactive (/ ", 12.4 years) and is believed to be the main source of the minute traces of 3He found in the atmosphere. It can be made artificially in nuclear reactors, for example, by the thermal neutron reaction, 6Li(/ ,a)3H, and is available for use as a tracer in studies of reaction mechanism. 

Ar production in the crust is small compared to the ambient background of atmosphere-derived Ar introduced into the crust dissolved in groundwater and is usually neglected. Although a small amount of muon-induced Ar occurs close to the surface, the principle route of production is the P-decay of C1 (Fontes et al. 1991 Hiinemohr 1989). C1 has a half-life of 3.01 x 10 yr and decays to Ar with a branching ratio, R, of 0.95. Cl is only produced in the crust by the thermal neutron reaction... 

Small amounts of the unstable gases Ar and thermal neutron reactions K(n,p) Ar... 

Takiue, M. and Ishikawa, H. 1978. Thermal neutron reaction cross section measurements for fourteen nuclides with a liquid scintillation spectrometer. Nucl Instrum Methods 148, 157-161. 

The production of a radionuclide becomes the basis of activation analysis methodology. In effect, a radioisotope is formed when the nuclei of any stable isotope is exposed to a source of neutrons or any other particle. The thermal neutron reaction upon Na-23 can be used to illustrate this mode of formation ... 

The theoretical methods have been previously reported and involve the computer codes ZUT and TUZ (resonance capture rates)/ THERMOS (thermal-neutron reaction rates), and MUFT (epithermal smooth reaction rates). Homogenization of the lattice was by flux-volume weighting in the fast and thermal-energy regions, and by volume weighting In the intermediate energy region. 

The chart is helpful when tracking down unknown radionuclides detected in a measurement to know what type of nuclear reaction may have been responsible for the production of the activity. In most cases, this will be a thermal neutron reaction resulting in activation by (n, 7), or if the target material is fissile, a fission reaction (n, f). 

The reaction is predominantly a thermal neutron reaction. To produce in N Reactor would require a special loading such as an s- l> or possibly the use of lithium control rods. Neither avenue has received serious study to date. 

Am undergo fission with thermal neutrons of these isotopes and Pu are the most important as they are most readily obtainable. Other heavy nuclei require fast neutrons to induce fission such neutrons are much more difficult to control into a self-sustaining chain-reaction. 

Thermal neutron absorption cross section. Simply designated cross section, it represents the ease with which a given nuclide can absorb a thermal neutron (energy less than or equal to 0.025 eV) and become a different nuclide. The cross section is given here in units of barns (1 barn = 10 cm ). If the mode of reaction is other than ( ,y), it is so indicated. 

In early 1941, 0.5 )-lg of Pu was produced (eqs. 3 and 4) and subjected to neutron bombardment (9) demonstrating that plutonium undergoes thermal neutron-induced fission with a cross section greater than that of U. In 1942, a self-sustaining chain reaction was induced by fissioning 235u... 

Beryllium has a high x-ray permeabiUty approximately seventeen times greater than that of aluminum. Natural beryUium contains 100% of the Be isotope. The principal isotopes and respective half-life are Be, 0.4 s Be, 53 d Be, 10 5 Be, stable Be, 2.5 x 10 yr. Beryllium can serve as a neutron source through either the (Oi,n) or (n,2n) reactions. Beryllium has alow (9 x 10 ° m°) absorption cross-section and a high (6 x 10 ° m°) scatter cross-section for thermal neutrons making it useful as a moderator and reflector in nuclear reactors (qv). Such appHcation has been limited, however, because of gas-producing reactions and the reactivity of beryUium toward high temperature water. 

Because the cytotoxic effects of the energetic lithium-7 and alpha particles are spaciaHy limited to a range of only about one-ceU diameter, the destmctive effects are confined to only one or two cells near the site of the event. Thus BNCT involves the selective deUvery of sufficiendy high concentrations of B-containing compounds to tumor sites followed by the irradiation of these sites with a beam of relatively nondestmctive thermal neutrons. The resulting cytotoxic reaction can then in theory destroy the tumor cells that are intimately associated with B target. 

The capture cross section of Li for this reaction using thermal neutrons is 930 x 10 (930 b) (43). AU of the experimental data available to the end of... 

This can result in a radioactive product from the A(n, t)A reaction where A is the stable element, n is a thermal neutron, A is the radioactive product of one atomic mass unit greater than A, and y is the prompt gamma ray resulting from the reaction. A is usually a beta and/or gamma emitter of reasonably long half-life. Where access to a nuclear reactor has been convenient, thermal neutron activation analysis has proven to be an extremely valuable nondestructive analytical tool and in many cases, the only method for performing specific analyses at high sensitivities... 

This NAA technique is based on the nuclear reactions 23Na(n,7)24Na and 41K(n/y)42K. Half-lives of the activated products are 15.0 hrs and 12.4 hrs, respectively. For Na analysis, the samples were irradiated in a specially designed thermal column to suppress the fast neutron reaction of 27Al(n,a)24Na which interferes with the reaction for Na. For K analysis, the proplnt samples were irradiated at a standard irradiation position of the reactor. For the Na irradiations, the neutron flux in the thermal column was in the order of 1010, whereas for the K assays it was approx 1012 neutrons/cm2-sec... 

Thermal neutron activation was selected for the radioassay via the soTi(n,7)s 1 Ti reaction. Since the half-life of 51Ti was 5.8 min, nondestructive NAA by gamma-ray spectrometry was used in lieu of chem sepn of Ti... 

Thru 1967, emphasis was given to the use of neutrons as the bombarding source of radiation. Almost all possible neutron reactions were considered including moderation of fast neutrons by hydrogen in the expl, thermal capture reactions, elastic and inelastic scattering of neutrons and neutron activation reactions. These neutron reactions are listed as follows ... 

The two Mossbauer levels of Pt, 99 keV and 130 keV, are populated by either EC of Au(fi/2 = 183 days) or isomeric transition of Pt(fi/2 = 4.1 days). Only a few authors, e.g., [323, 324] reported on the use of Pt, which is produced by thermal neutron activation of " Pt via " Pt(n, y) Pt. The source used in the early measurements by Harris et al. [322, 325] was carrier-free Au diffused into platinum metal. Walcher [326] irradiated natural platinum metal with deuterons to obtain the parent nuclide Au by (d, xn) reactions. After the decay of short-lived isotopes, especially Au(fi/2 = 6.18 days), Au was extracted with ethyl acetate, and the Au/Pt source prepared by induction melting. Buym and Grodzins [323] made use of (a, xn) reactions when bombarding natural iridium with... 

The cross sections of the thermal neutron capture reaction of 186W and 187W are... 

---