Epithermal neutron irradiation

The several polymeric metal carbonyls studied have led to some surprisingly high yields [e.g., Fe3(CO),2 and Ruj(CO)j2 in Table IV] but to no substantiated mechanisms. The 17% yield of Fe3(CO),2 in neutron-irradiated Fe(CO)j was interpreted as a reaction of Fe(CO)4 with the Fe(CO)5, but no further evidence is available. The study of Mn2(CO),o has been fruitful (44, 46). The insensitivity of the parent yield MnMn(CO),o to heat indicates that the molecule is formed by a reaction quite early in the sequence, perhaps epithermal. The discovery (46) of a species which reacts rapidly with I2 and exchanges with IMn(CO)5 led to the conclusion that the Mn(CO)5 radical is produced prominently (4.5%) by nuclear reactions in the solid decacarbonyl. The availability of this labeled Mn(CO)5 has made possible several interesting observations about the exchange properties of this radical in the solid (45) and in solution (42). 

In general, activation analysis relies on the use of standards that are irradiated under the same conditions and in the same position, and are also measured under the same conditions. Monoelement standards contain a known amount of one element. If they are applied to the evaluation of other elements the ratio of the cross sections o x/o s under the special conditions of irradiation and the ratio H /Hs of the relative abundances of the decay processes that are measured must be known (subscript x is for the sample and subscript s for the standard). Knowledge of the ratio o x/o s may cause problems, because the cross sections may vary drastically with the energy of the projectiles, for instance in the energy range of epithermal neutrons. These problems are not encountered with multielement standards that contain all the elements to be determined. However, the preparation of such multielement standards may be time-consuming. 

A maximum specific activity of approximately 850 GBq/mg was achieved when irradiation was carried out at a thermal neutron flux of 1 x 10 n-cni -s for 21 d, which corresponds to around 21% of the maximum achievable specific activity. Tlie specific activity of the Lu obtained was significantly higher than the theoretically calculated value under the irradiation conditions employed (7.9 at.%), accounting for only thermal neutron capture. This could perhaps be attributed to the contribution from epithermal neutrons (resonance integral 1087 b), which is not accounted for in theoretical calculations. 

If a sample contains N nuclei of a particular stable isotope, the rate of formation of its (n,y) product nuclei is Nc in which is the neutron flux density of the thermal and epithermal neutron flux density in n-cm -s (O = Oth + epi) and a the (n.y) cross section (generally expressed in barns, one barn equalling 10 cm ) of the target nuclei in the neutron spectrum in which the sample is irradiated a = + lo epi/ ) ... 

However, irradiation of biological materials with thermal and epithermal neutrons also results in high background activities from Na, C1, and activation... 

Setup for a lateral BNCT brain Irradiation ushg the fission converter-based epithermal neutron beam at the Massachusetts Institute of Technology with a 12 cm diameter aperture. 

In Eq. (30.38), the term Qo(a)//in the factor (1 + Qo( )//) corrects for activation by epithermal neutrons. This factor is the heart of the ko method and was the key innovation that made the method possible. It was necessary to predict, with high accuracy, the relative reaction rates for two different (n,y) reactions in any reactor neutron spectrum. For each reaction, the Qo value, the ratio of the resonance integral to the thermal neutron activation cross section, and / the ratio of thermal flux to epithermal flux for the irradiation channel used, is needed. ... 

Many methods have been developed to measure /and a. They all involve the activation and counting of a number of nuclides having a range of Qo values and mean resonance energies. The most accurate measurements of a use irradiations under cadmium cover to activate only with epithermal neutrons. However, many laboratories may not require such high accuracy or may not be permitted to irradiate under cadmium cover. Bare irradiations have been shown to give sufficient accuracy if carefully done. Since the parameters are determined by subtraction of the thermal neutron-produced activity from the total activity- two possibly similar quantities -accurate element masses, peak areas, and detection efficiencies are needed. The minimum number of monitors that need to be irradiated for the simultaneous determination of the thermal neutron flux, the factor /, and a. is three. 

One of the most convenient trios of monitor elements is Cr, Au, and Mo (Koster-Ammerlaan et al. 2008). The Cr(n,y) Cr reaction has a low Qo value and is primarily produced by thermal neutrons. The Au(n,y) Au reaction is produced by thermal and low-energy epithermal neutrons, and the Mo(n,y) Mo reaction is produced by thermal and high-energy epithermal neutrons. Along with samples to be analyzed, the irradiation of a pellet containing weU-known quantities of Cr, Au, and Mo, typically 500, 4, and 1,000 pg, respectively, gives the thermal neutron flux, /and a after a simple calculation. 

The NAA technique makes use of the slow neutrons. However, the epithermal and fast neutrons may also be used for the activation. An NAA technique that employs only epithermal neutrons to induce (n, y) reactions by irradiating the samples being analyzed inside either cadmium or boron shields is called epithermal neutron activation analysis (ENAA). An NAA technique that employs nuclear reactions induced by fast neutrons is called fast neutron activation analysis (FNAA). 

The term PCNAA is used when preconcentration precedes the neutron activation while if epithermal neutrons are used to excite the sample the acronym given is ENAA. The monitoring of the delayed neutrons emitted after excitation is termed DNAA. All these NAA procedures are nondestructive techniques used for characterizing solid (and in some cases also liquid) samples. However, a neutron source and a suitable detector are required and the sample can become quite radioactive after irradiation. The sensitivity of NAA techniques varies widely among different elements and sample preparation and post-irradiation methods employed. Several specific examples of NAA application for analysis of uranium in different matrices will be presented in the appropriate chapters. 

The INAA technique generally involves a neutron irradiation followed by a decay period of several minutes to many days. In some cases, cyclic INAA (CINAA) is employed whereby a sample is irradiated for a short time, quickly transferred to a detector and counted for a short time. This process of irradiation-transfer-counting is repeated for an optimum number of cycles. The INAA technique uses reactor neutrons that are a combination of thermal and epithermal neutrons. When mostly epithermal neutrons are used for irradiations, the technique is called epithermal INAA (EINAA). 

Thermal neutrons are desired at the tumour location because the B interaction probability is much higher with slower neutrons. Therefore, surface or shallow tumours can be irradiated with thermal neutrons, while those at a depth of a few centimetres can be irradiated with epithermal neutrons which then become thermalized by the overlying tissue. Thermal neutrons are also useful for research involving cell cultures or small animal irradiations. 

The current discussion will focus on epithermal beams, since that is where the research appears to be leading. The commonly accepted definition of epithermal neutrons as being those between 0.5 eV and 10 keV is used. Current experience shows that a desirable minimum beam intensity would be 10 epithermal neutrons cm s V Beams of about half this are useable, but result in rather long irradiation times. 

Samples can be activated with epithermal neutrons, those in the process of being moderated to thermal energies, by irradiating in a Cd container which absorbs the thermal neutrons. Bnuifelt and Steinnes (1973) have used epithermal neutron... 

To deliver the requisite thermal neutrons into deep lesions, epithermal neutrons are currently the standard radiation source for BNCT. Epithermal neutrons are thermalized in the scalp or at the brain surface, the thermal neutron dose reaching its maximum intensity at a depth of 2 cm, almost equal to the total thickness of scalp and skuU (Figure 11.6) [6]. Thus, thermal neutron doses are acquired in the highest concentration at the brain surface when the epithermal neutrons are irradiated onto the scalp surface [7]. This modality allows BNCT without craniotomy. To enable thermal neutrons to penetrate as deeply as possible, however, deeply seated tumors can be treated by BNCT after a craniotomy to expose the brain. 

---