Neutron irradiation reactions limitations

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. 

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. 

In PIGE the y-emission is usually prompt. If very low amounts of trace elements have to be detected it can be advantageous to use a delayed decay. In this case, the technique is called charged particle activation (CPA) and is an analogue to neutron activation analysis (NAA). It has the advantage that the prompt background from interfering reactions is completely removed as irradiation and analysis are completely separated in time. This also allows to remove external contaminants in the short time between irradiation and measurement which further improves detection limits. A comprehensive description of the technique can be found in the ion beam analysis handbook [2], For 19F CPA is conceivable in special cases via the 19F(d,dn)18F reaction. However, we have found only one application in the literature [64],... 

Reactor neutrons are most frequently used for activation analysis, because they are available in high flux densities. Moreover, for most elements the cross section of (n,y) reactions is relatively high. On the assumption that an activity of lOBq allows quantitative determination, the lower limits of determination by (n,y) reactions at a thermal neutron flux density of lO cm s are listed in Table 17.2 for a large number of elements and two irradiation times (1 h and 1 week). Detection limits of the order of 10 to g/g are, in general, not available by other analytical methods. 

In vivo analysis of the human body by means of activation spectrometry requires irradiation with energetic neutrons in order to reduce excessive absorption in the outer layers by thermal neutron capture. At the same time the radiation dose must be as low as possible with an upper limit of the order of 10 mSv under these circumstances it is found that better results for the major elements are found [7] when y-spectrometry is carried out both during and immediately after irradiation, so that the irradiation emitted directly as a result of the nuclear reaction is measured. 

A meaningful assessment of potential changes in the properties of the materials measured in these examinations is only possible when the neutron fluence the sample had been exposed to in the course of reactor irradiation is known. This neutron fluence can be determined by measurement of the radioactivity of specific radionuclides which have been produced by neutron reactions. However, out of the great number of radionuclides produced, only a few can be reasonably applied as fluence monitors, since there are several limitations which have to be taken into account ... 

Frequently the nuclear reaction Ni (n,p) Co is used for determination of the fast neutron fluence, but it has two severe drawbacks when compared to the Fe-Mn reaction described above The rather short halflife of Co of 71.3 days limits the application of this reaction to irradiation times of less than 150 days, even when the irradiation history of the sample is taken into account. In addition, the complex nature of the nuclear reaction requires expensive calculations when accurate results are required. On the other hand, the intense y ray of Co at 0.81 MeV (transition probability 99.5%) can be easily measured using a Ge detector, in most cases making chemical separation work unnecessary. 

Major issues of radiation effects on V-aUoys are radiation embrittlement at relatively low temperature, and irradiation creep at intermediate temperature. Void swelling is known to be quite small if the alloy contains Ti. He embrittlement is a key issue determining the high-temperature operation limit in fusion neutron environments where 5—10 appm He are produced by transmutation during the irradiation to 1 dpa. However this may be a minor issue for fission neutron environments where the production rate is much lower because the cross-section of He-producing reactions is small when the neutron energy is below 10 MeV. 

The ternary Am- Cm-Be source (the ABC source) may find increasing application, since it may be possible to prepare sources with outputs of up to 5 X10 ° s This type of source is produced by reactor irradiation of the conventional Am-Be source. Wing and Wahlgren have estimated the optimum sensitivities for 60 elements using an ABC source with a fast-neutron output of 5 X10 s S giving fluxes of 1.4x 10 ncm s" These authors used this source for the measurement of F, by the F(n,a) N reaction with a lower limit of detection of 0.4 mg of F. The precision of the method was good and the measurement rapid. The sensitivity compared favourably with methods using accelerator sources of neutrons. 

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