Nuclear resonance absorption

Soreq [41] has developed a nitrogen-rich (31% by weight) N-based ionization detector liquid scintillator based on di-methyl-tetrazole that is resonance sensitive (24% efficient for resonant photons) and is well suited for the explosive detection application. 

In addition to the 9.17 MeV photon transmission, the NRA approach needs a background gamma measurement to assist in compensating for the mass-dependent attenuation (non-resonant absorption) of the photons in the cargo. Prototype NRA 

Another disadvantage with NRA is the requirement for very advanced high current accelerator. This accelerator wih have all of the disadvantages (large size, large weight, radiation shielding) associated with the neutron-based accelerators. 

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Fig. 2.1 Nuclear resonance absorption of y-rays (Mossbauer effect) for nuclei with Z protons and N neutrons. The top left part shows the population of the excited state of the emitter by the radioactive decay of a mother isotope (Z, N ) via a- or P-emission, or K-capture (depending on the isotope). The right part shows the de-excitation of the absorber by re-emission of a y-photon or by radiationless emission of a conversion electron (thin arrows labeled y and e , respectively)...

Hence, nuclear resonance absorption of y-photons (the Mbssbauer effect) is not possible between free atoms (at rest) because of the energy loss by recoil. The deficiency in y-energy is two times the recoil energy, 2Er, which in the case of Fe is about 10 times larger than the natural line width F of the nuclear levels involved (Fig. 2.4). 

Fig. 2.6 Schematic illustration of a Mossbauer transmission experiment in five steps. The Absorption bars indicate the strength of recoilless nuclear resonant absorption as determined by the overlap of emission and absorption lines when the emission line is shifted by Doppler modulation (velocities Uj,. .., 1)5). The transmission spectrum T v) is usually normalized to the transmission T oo) observed for v oo by dividing T(v)IT(oo). Experimental details are found in Chap. 3...

Nuclear resonance absorption for the 136 keV transition has been established by Steiner et al. [174]. The authors used a metal source and an absorber of metallic tantalum to determine the mean lifetime of the 136 keV level from the experimental line width ( 52.5 mm s for zero effective absorber thickness) and found a value of 55 ps. This has been the only report so far on the use of the 136 keV excited state of Ta for Mossbauer experiments. 

There are two iridium isotopes, ir and Ir, suitable for Mossbauer spectroscopy. Each of them possesses two nuclear transitions with which nuclear resonance absorption has been observed. Figure 7.58 (from [266]) shows the (simplified) nuclear decay schemes for both iridium Mossbauer isotopes the Mossbauer transitions are marked therein with bold arrows. The relevant nuclear data known to date for the four Mossbauer transitions are collected in Table 7.1 at the end of the book. 

It is a matter of historical interest that Mossbauer spectroscopy has its deepest root in the 129.4 keV transition line of lr, for which R.L. Mossbauer established recoilless nuclear resonance absorption for the first time while he was working on his thesis under Prof. Maier-Leibnitz at Heidelberg [267]. But this nuclear transition is, by far, not the easiest one among the four iridium Mossbauer transitions to use for solid-state applications the 129 keV excited state is rather short-lived (fi/2 = 90 ps) and consequently the line width is very broad. The 73 keV transition line of lr with the lowest transition energy and the narrowest natural line width (0.60 mm s ) fulfills best the practical requirements and therefore is, of all four iridium transitions, most often (in about 90% of all reports published on Ir Mossbauer spectroscopy) used in studying electronic stractures, bond properties, and magnetism. 

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

The Development of a Prototype High-Explosive Detection System Based on Nuclear Resonance Absorption (Project OPEB-A), Summary report, Soreq Nuclear Research Center., December 1993. 

Morgado et al., The Effects of Proton Beam Quality on the Production of Gamma Rays For Nuclear Resonance Absorption in Nitrogen, in Substance Detection Systems, G.L. Harding, R. C. Lanza, and L.J. Myers, Editors, Proceedings SPIE 2092, (1994) 503. 

R. E. Morgado, G. Amone, C. C. CappieUo, S. D. Gardner, C. L. Hollas, L. E. Ussery, J. M. White, J. D. Zahrt, R. A. Krauss, Prototype Explosives-Detection System Based on Nuclear Resonance Absorption in Nitrogen, Los Alamos National Laboratory, Report LA-12776-MS, Los Alamos, NM, June 1994. 

D. Vartsky, G. Engler, M. B. Goldberg, R. A. Krauss, A Method for Detection of Explosives Based on Nuclear Resonance Absorption of Gamma Rays in 14N, SPIE, Vol. 2092, Substance Detection Systems (1993) 307. 

Mossbauer spectroscopy, also called recoil-free nuclear resonance absorption, depends upon resonant absorption of y-rays emitted by a radioactive source by atomic nuclei.120 The phenomenon was initially difficult to observe, but the German physicist Mossbauer devised a way in which to record the absorption of a quantum of energy equal to the difference in two energy states of the atomic nucleus. The method depends upon a Doppler effect observed when the sample or source moves. Consequently, Mossbauer spectra, such as that in Fig. 16-18, are plots of absorp-... 

Recoilless nuclear resonance absorption of y-radiation (used by MoBbauer in his Nobel lecture 1961)... 

Pake GE (1948) Nuclear resonance absorption in hydrated crystals fine stracture of the proton line. J Chem Phys 16 327... 

In the terminology of spectroscopy, resonance is the condition in which the energy state of the incident radiation is identical with that of the absorbing atoms, molecules or other chemical entities. Resonance is applied in various types of instrumental analysis such as nuclear resonance absorption and nuclearmagneticresonance. See absorption spectroscopy. 

Like hydride complexes in general, the carbonyl hydrides generally exhibit sharp M—H stretching bands in the infrared and proton nuclear resonance absorptions at very high % values as shown in Table 22-3. 

It will hardly be necessary to recall that it was with the 129 4-keV y-transi-tion in Ir that R. L. Mbssbauer first demonstrated nuclear resonance absorption [64]. The source used was 16-day Os (see Fig. 16.22 for decay scheme) and the absorber was iridium metal. The transmission of the y-rays decreased unexpectedly as the temperature was lowered from 370 K to 90 K. Subsequently he initiated the use of velocity scanning [65, 66], and derived the excited-state half-life as Ij = 0 099 ns. 

The interference of the photoelectric effect and nuclear resonance absorption which gave a dispersion term to the line-shape of Ta (see p. 508) also occurs in the 97-4-keV El transition of Eu. A small asymmetry has been... 

The conditions for such a nuclear resonance absorption are very stringent. Using the Heisenberg relationship (4.66) we can estimate the half-value width of the 129 keV peak to be 5 X 10 eV. We can also use relation (4.34) to calculate the iridium atom recoil energy to be 46 X 10 eV. Thus the 7-my leaves the source with an energy of (129 x 10 ... 

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