Prompt photons

Wiggins et al. [456] used neutrons from the thermal column of a 10 kW pool-type research reactor and from a 120 pg Cf source to study the prompt photon emission resulting from neutron capture in magnesium nodules (ter-romanganese oxides) from the ocean floor. Spectra were recorded with a Ce(Ii) detector and a 1024-channel analyser. Complex spectra were obtained by irradiation of seawater, but it was possible to detect and estimate manganese in nodules in a simulated marine environment by means of the peaks at 7.00, 6.55, 6.22, and 6.04 pV. 

In recent years a number of other non-intrusive techniques for determining the contents and status of unopened munitions have been developed. These include neutron interrogation techniques such as neutron-induced prompt photon spectroscopy and hydrogen concentration measurement, and ultrasonic pulse echo and acoustic resonance techniques. Equipment based on the use of neutron-induced prompt photon spectroscopy has been fielded and successfully used by US EOD teams on many occasions over the last two years. This equipment, which is based on the analysis of photons emitted as the result of the interaction of neutrons with the chemical elements present in the munition and its contents, makes it possible to identify the presence or absence of key elements such as phosphorus, sulphur, chlorine, fluorine and arsenic within the munition. Clearly this information not only helps to identify the presence of a chemical fill, but in many circumstances also means that it is possible to identify the fill. 

The prompt photon associated with the emission of the positron is used to trigger a timing circuit. It was found that the appearance of the annihilation photons occurred after a certain delay time. This led to the model that before annihilation. 

Neutron induced prompt photon spectroscopy (NIPPS)... 

Figure 17 Neutron Induced Prompt Photon Spectroscopy...

The radiation that results from nuclear de-excitation has to be distinguished from the prompt radiation, i.e., from the photons that pass through the sample without interaction and from those that are scattered by the electrons. This is achieved if three characteristic times (Ati, At2, and t) are properly related ... 

The detection probability for a given trajectory depends on the fragment orientation (its Mj value) and the nature of the probe transition. All of these images were obtained via the two-photon Ilg XAS) transition. Five rotational branches are thus possible O, P, Q, R and S. The amplitudes for each of these two-photon transitions can be obtained from a sum of paired, Mj-dependent, one-photon amplitudes.37 The O branch, for example, consists of a contribution from a parallel P-type transition to a 7A virtual state, followed by a perpendicular P-type transition to the final 1ffs Rydberg (which is assumed to be ionized promptly). The product of those two transition amplitudes must be summed with another product in which the first transition is perpendicular and the second is parallel. The P and R branches consist of four contributions each and the Q branch has six such terms in its transition amplitude. The required one-photon amplitudes are taken from Ref. 37. 

The yields of eh (Oe) estimated from the prompt (At=30 ns) signal amplitudes at 650 nm, increase nonlinearly with the laser power (f) as shown in Fig. 2. The value of =1.75 0.1 estimated from the power dependence of Oe vs f plots is typical of a two-photon ionization process [43]. 

The product nuclei as initially formed are highly unstable isotopes and emit delayed neutrons as well as electrons and gamma photons while settling down into their stable configurations, which ate usually isotopes of different elements from those first formed. The neutrons, both prompt and delayed, continue the reaction by encountering other fissionable nuclei... 

Since j> is independent of I , the ratio of the intensity of P-type delayed emission to that of prompt fluorescence should show linear dependence on the intensity of absorption. The square law dependence indicates the necessity of two photons for the act of delayed emission and is hence known as biphotonic process. It has been observed in fluid solutions of many compounds and also in the vapour state. 

Prompt y-ray emission competes with or follows the last stages of prompt neutron emission. These photons are emitted in times from 10 15-10 7s. Typical y-ray multiplicities of 7-10 photons/fission are observed. These photons, as indicated earlier, cany away 7.5 MeV. This y-ray yield is considerably larger than one would predict if y-ray emission followed neutron emission instead of competing with it. Because of the significant angular momentum of the fission fragments ( 7-10 h) even in spontaneous fission, photon emission can compete with neutron emission. The emitted y rays are mostly dipole radiation with some significant admixture of quadrupole radiation, due to stretched El transitions (J/= 7, — 2). 

The fact that the single photon transitions require so little power prompts us to consider two photon transitions. Consider the Na 16d — 16g transition via the virtual intermediate 16f state which is detuned from the real intermediate 16f state as shown by the inset of Fig. 16.3. If the detuning between the real and virtual intermediate states is A and the matrix elements between the real states arefxx and fi2, the expression analogous to Eq. (16.5) for a two photon transition is... 

An a-y coincidence experiment was performed using a cooled Si(Li) detector for the detection of photons and a Si detector for the detection of a-particles. Three parameter events were collected on tape and one dimensional spectra were later generated in coincidence with various gates. The spectra showed that the and a3Q are in prompt coincidence with L X-rays and the delay occurs at the 27.4 keV level. The analysis of the time spectrum between the group and the 27.4 keV photopeak gave a half-life of 38.3 - 0.3 ns, in agreement with previous measurements. 

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