Activation analysis with 14 MeV neutrons

The principles of and the apparatus used for activation analysis with 14 MeV neutrons (14 MeV NAA) have been described in detail e.g. by Nargolwalla and Przybylowicz (4), by Adams et al. (5), by De Soete et al. 

In a neutron generator, deuterons accelerated in a high vacuum to an energy of 100-600 keV bombard a tritium target made e.g. of tritiated titanium on a copper backing. The fusion reaction 

Because of the flux gradients, it is practically impossible to irradiate a stationary sample and a standard simultaneously with the same neutron flux. Therefore, e.g. the following solutions can be adopted  

Because of target depletion and of limited life of sealed neutron tubes, neutron generators are most suitable for short irradiations, hence for the formation of short-lives isotopes. This implies a fast transfer system, usually pneumatic. 

Hoste et al. (14) described a system that allows the irradiation and the subsequent measurement of a sample and a standard under strictly constant geometrical conditions. 

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Although activation analysis with 14 MeV neutrons can also be used e.g. for the determination of nitrogen in metals (184), using the N(n,2n) N nuclear reaction, this method has been most useful for the determination of oxygen. 

The oxygen concentration in titanium, zirconium and their alloys is usually fairly high (several hundred to several thousand Mg/g). Activation analysis with 14 MeV neutrons can thus be used for an accurate determination of oxygen in those metals. Less attention has therefore been given to the determination of oxygen by charged particle activation analysis. 

The most common sources are based on the 3H(d, n) reaction. Deuterons are accelerated to 150 keV with currents 2.5 mA and strike a tritium target. They produce 2 x 1011 of 14-MeV neutrons/s under these conditions. The neutrons produced are widely used in fast neutron activation analysis for the determination of light elements. The tritium targets are typically metals such as Ti, which have been loaded with titanium tritide. The accelerators are usually small Cockcroft-Walton machines or small sealed-tube devices where the ion source and accelerator structure are combined to produce a less expensive device with neutron yields 108/s. 

Activation analysis is based on the production of radioactive nuclides by means of induced nuclear reactions on naturally occurring isotopes of the element to be determined in the sample. Although irradiations with charged particles and photons have been used in special cases, irradiation with reactor thermal neutrons or 14 MeV neutrons produced by Cockcroft-Walton type accelerators are most commonly used because of their availability and their high probability of nuclear reaction (cross section). The fundamental equation of activation analysis is given below ... 

It is obvious, therefore, that 14 MeV neutron activation analysis can not compete with thermal neutron activation analysis as a technique for trace element analysis. In simple matrices, however, the rapid and non-destructive nature of the technique recommends its use for routine analysis of large numbers of samples for elemental abundances at the one milligram level, or above. It is unfortunate that the element carbon can not be determined by this technique. The nuclear reaction 12C(n, 2n)1 C which would be of great analytical importance is endoergic to the extent of nearly 19 MeV. This reaction is obviously not energetically possible using the 14.7 MeV neutrons produced by the 2H(3H,w)4He reaction commonly employed in most neutron generators. 

Discussions of errors associated with the technique of activation analysis in general may be found in many of the books and monographs referenced in the introduction to this paper. Interferences unique to 14 MeV neutron activation techniques have been reviewed by Mathur and Oldham 42> and a discussion of precision has been published by Mott and Orange 43>. 

The use of 14 MeV neutron activation principally for major elements, Ge(Li) detectors for trace elements following thermal neutron irradiations, and gamma-gamma coincidence techniques for positron or cascade gamma-ray emitters as discussed in the previous sections, provide the analyst with powerful tools for devising schemes for non-destructive analysis. A few additional activation techniques which may be useful in special applications are discussed briefly below. In most of these cases rather sophisticated instrumentation is required. It is unlikely, therefore, that these techniques will be in routine use in a facility devoted principally to analytical applications. In some cases, however, arrangements may be made for part time use of a more extensive nuclear facility for a specific analytical problem. 

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. 

Other major shale constituents such as C, H, N, and S are determined by thermal decomposition and instrumental detection methods. Oxygen is determined by 14 MeV neutron activation analysis. Parr or Leco BTU bomb combustion and subsequent ion chromatographic determination is used for halogens, sulfate and nitrate. Ion chromatography is also suitable for anionic characterization of shale process waters. Two analytical procedures for oil shales should be used with caution. Kjeldahl nitrogen procedure has been found to give reproducible but considerably low results for certain oil... 

Ehmann, W. D. Nondestructive Techniques in Activation Analysis, Fortsch. Chem. Forsch., 74(1), 49 (1970). This is a general article on the technique of nondestructive neutron activation analysis with major emphasis on 14-MeV neutrons. Practical aspects of this area as well as some advanced developments are discussed. 

Oxygen. Analyses are ordinarily made by vacuum fusion with Fe or Pt [22,23,27] or by inert gas fusion [28]. The sample is dissolved in the molten metal saturated with carbon in a graphite crucible at 1900°C and the evolved CO converted to CO2 in a CuO column and measured gas-volumetrically. More reproducible results are said to be obtained by neutron activation analysis for oxygen. With this method the sample is exposed a few minutes to 14 MeV neutrons to activate the oxygen by the nuclear reaction 0(n,Y) 0 and the amount of oxygen is obtained from the 0 activity having the half life of ty = 29.1 s [29] by comparison with that of a standard. The technique of isotopic dilution which has been used to analyze for oxygen in Ti should be applicable too [30]. 

Cu has been determined in iron and steel using the Cu(n,2n) Cu reaction, and several other metallic elements have been measured in scrap solders.The determination of A1 and Cl in composite propellents was accomplished by a mixed fast/thermal activation using partially moderated 14 MeV neutrons in the Al(n,a) Na and Cl(n,y) Cl reactions. The elements Si, Cl, K, P, Ca, and A1 have all been measured in biological material by 14 MeV n.a.a. using NaI(Tl) spectrometry in conjunction with decay curve analysis. Ge(Li) spectrometry would have been advantageous. 

The results of the sandwich technique agree very well with those obtained by 14 MeV neutron activation analysis, and are furthermore characterized by a lower standard deviation. 

Classical methods such as reducing fusion or 14 MeV neutron activation analysis are only able, under optimal circumstances, to carry out routine control analyses. In most cases this means "YES/NO-analyses" with a sensitivity of approximately 0.5 ig/g. Activation analysis with photons or charged particles allows to determine concentrations of 0.1 tg/g or below. These developments led... 

The same conclusions are valid for zircaloy. A batch of this material (diameter 13 mm) has been recently analysed within BCR, with the results listed in Table VII-30. 14 MeV neutron activation analysis was not applied, due to the limited diameter of the material. 

Recommended reactions for neutron activation analysis (NAA) with small neutron generators note that (0,7) relates to thermal energy other reactions to E = 14.5 MeV. X stands for X-rays, -annih. for 511 keV annihilation radiation... 

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