Prompt radiation analysis

Figure 5 A multidetector setup used in combination with a nuclear microprobe. Types of spectra obtained are indicated. PIGE, particle-induced y-ray emission and PBA, prompt radiation analysis. See text for further explanation.

In addition to NAA, neutrons are widely used in prompt radiation analysis for the determination of concentration and spatial distribution of elements in different matrices. For example, a track-etched detector (LR-115, Makrofol KG, CR 39, mica) placed on the polished surface of a sample is irradiated with fast or thermal neutrons then etched with a suitable chemical to deduce the concentration profiles from the track distributions. This method can also be used for the detection of suspended and dissolved U, Th, and Pu in water by (n,f) reactions N in polymers by the N(n,p) C reaction B and Li in semiconductors or glasses by the B(n,a) Li and Li(n,ot) H reactions, respectively. For the detection of fission fragments the use of mica is recommended. 

There are many more new fields in the utilization of neutrons produced mainly by small D-D and D-T accelerators (IAEA 2000). Some typical fields of activation and prompt radiation analysis are summarized in Table 32.1. 

Prompt activation analysis (Erdtmann and Petri, 1986 Alfassi, 1990) uses the prompt radiation accompanying a nuclear reaction for determining elemental or isotopic concentrations. The variety of prompt methods is large because a sample can be irradiated with various particles - neutrons, charged particles or gamma-rays. Prompt activation analysis permits the determination of several elements - about 17 elements in environmental matrices (Germani et al., 1980) - but most analysis are used for the determination of light elements (H, He, Li, B, C, N, Si, S, Cl) as well of Cd and Gd. 

In neutron activation analysis, an activation of a sample material is accomplished by placing the sample in some position within the neutron environment. At the time the sample is exposed to neutrons, a compound nucleus is formed as the result of the interaction of a neutron with the nucleus of a stable isotope of the element being determined. The compound nucleus i.e. the end-product of an excitation process caused by both the kinetic and binding energy of the neutron with the nucleus, instantaneously loses its excess energy by a transformation to a more stable isotope by emitting prompt radiations. As a result of this event, another stable nuclide or a radioactive isotope is formed. This radioisotope then becomes the activation, or isotopic, indicator of the element of interest. 

For short half-lives, C approaches zero and the counting increases linearly with the measurement time. This is automatically the case when prompt radiation is detected instead of decay radiation. For this reason, prompt gamma activation analysis is inherently more sensitive than delayed gamma activation analysis. 

Using a pulsed cyclotron, Chen and Fremlin applied deuteron activation to the measurement of F, N, Cl, and Na. The technique involved cyclic irradiation of the sample using a 30 ms cyclotron pulse, 1 ms delay (for the decay of the activation pulse and prompt radiation) and 30 ms count period using either NaI(Tl) or Ge(Li) detectors. For 5—10 min measurement periods, 5—10% accuracy was attained for the analysis of F and Na at the 100 p.p,m. level. 

C. Radiation Injury Severe. These casualties are judged to have received a radiation dose that is potentially fatal. Nausea and vomiting will be almost universal for persons in this group. The prodromal phase may also include prompt explosive bloody diarrhea, significant hypotension, and signs of necrologic injury. These patients should be sorted according to the availability of resources. Patients should receive symptomatic care. Lymphocyte analysis is necessary to support this classification. 

The history of neutron activation analysis goes back to the middle of the 1930s when it was first described by G. Hevesy and H. Levi at the Niels Bohr Institute in Copenhagen. The principle of the technique is that elements can be made radioactive by exposure to neutron irradiation. Two types of physiological processes are associated with this activation one prompt and one delayed. Classically, neutron activation analysis is based on the detection of the delayed event, viz. the characteristic radiation emitted during the decay -with a specific half-life (ti/a) - of the unstable nuclei formed by (n,y) reaction. 

The quality control (QC) tests discussed in Sections 10.5 and 11.2.9 are integral parts of QA designed to check results. Some QC measures are prompt indicators that warn of problem occurrence at the time of analysis others are delayed indicators that require backtracking to And when a problem first arose. Control charts for radiation detector operation are an example of a prompt indicator of reliability. Records of deviations from the norm in an analysis or a measurement may also be prompt indicators if immediately considered. Periodic blank, blind, and replicate analyses, especially interlaboratory comparisons, are delayed indicators for which results may not be available for days or weeks after a problem has arisen. Review and assessment of compiled data are delayed indicators of information quality. 

Review of the process prompted by analyst or operator concerns should involve the analyst, the supervisor, and possibly a specialist. This review may be able to distinguish among possible causes in chemical analysis such as matrix difference, method instability, bad reagents, or analyst error. In radiation detection, source problems, detector malfunction, and data analysis must be distinguished. The discussion should focus on what the analyst or operator remembers about the measurement series in question, in contrast to records for similar analyses this should help determine when the problem was first observed and the differences in the process since then. 

In a full analysis i.e., the prompt as well as delayed y-emission analysis, the short-lived nuclides are determined first. Actually the most intense y-radiations are measured first and then observation about their decrease of intensity is made. (This will happen for y-radiations of those isotopes which have short half-lives). Using a fast rabbit system, each sample is irradiated separately (but together with a comparator in order to calculate the neutron flux) for 5-30 s. Sample and comparator are measured separately after a waiting time from seconds (the element selenium has a nuclide with a half-life time of 17.5 s and thus needs to be measured as quickly after irradiation as possible) to as much as 20 min. After the analysis has been completed for all samples, a waiting time of 5-7 days is required before irradiating them again in order to determine the other elements with longer half-life times. 

An alternative to transferring the sample from irradiator to detector, either manually or by the process stream, is to measure the capture y-radiation emitted by the sample. The instantaneous measurement should be less dependent on flow rate when used for on-stream applications, and may therefore give more precise results. Tiwari et used a 2.5 x 10 n s Am-Be source for the off-line measurement of N in organic materials using the 10.83 MeV prompt y-ray from the N thermal-neutron capture reaction. The possible use of Pu-Be neutron sources for in situ analysis of rocks, using either NaI(Tl) or Ge(Li) detectors for prompt y-radiation, has been discussed with particular reference to extraterrestrial applications. ... 

Prompt Mefliods of Instrumental Analysis.— Prompt methods of analysis are based on the measurement of the radiations emitted during a nuclear reaction, instead of measurement of the radiations emitted by the decay of the product of the reaction, as in conventional activation analysis (decay analysis). The sensitivities of prompt methods are independent of the half-life of the product isotopes and are, therefore, intrinsically more sensitive than decay methods,... 

Most prompt methods of analysis depend upon the measurement of thermal neutron capture radiation using reactor sources of neutrons. In spite of the large potential increase in sensitivity, relative to conventional reactor activation, practical application of the prompt method is limited by the low neutron fluxes available in neutron beams. It is seldom possible to exceed a thermal neutron flux of 10 cm s". Henkelmann, in his assessment of prompt y-ray analysis, concludes that (assuming a neutron flux ratio of 10 between decay and prompt methods) prompt methods only offer increased sensitivity... 

Macey and Gilboy have investigated the prompt measurement of radiation from the 0(d,p) 0, N(d,n) 0, and N(d,p) N reactions and estimated the limits of detection to be 3 and 4 /fg cm" for O and N, respectively, in gases. A wider application to solid samples would be useful. A study of the determination of F by prompt y-radiation from proton bombardment reports limits of detection of 20 p.p.m. in solid and liquid samples. In both of the above methods, sodium iodide spectrometry was found to be adequate in view of the specificity of the analysis. 

---