Prompt neutron yields

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). 

Glendenin (87) attempted to explain the high yields of the 133 and 134 mass chains by assuming that nuclei with 83 neutrons (which has already emitted the usual number of prompt neutrons) would have a high probability of boiling off a prompt neutron to form the most stable 82 neutron configuration rather than emit or y rays as in the ordinary case. According to this scheme, the 134 mass chain would be increased by the reaction... 

Abstract This chapter first gives a survey on the history of the discovery of nuclear fission. It briefly presents the liquid-drop and shell models and their application to the fission process. The most important quantities accessible to experimental determination such as mass yields, nuclear charge distribution, prompt neutron emission, kinetic energy distribution, ternary fragment yields, angular distributions, and properties of fission isomers are presented as well as the instrumentation and techniques used for their measurement. The contribution concentrates on the fundamental aspects of nuclear fission. The practical aspects of nuclear fission are discussed in O Chap. 57 of Vol. 6. 

In addition, the physical measurements (after some corrections for prompt neutron emission) allow one to obtain fragment mass distributions (prior to prompt neutron emission). A comparison of these fission fragment yield curves with fission product yield curves makes it possible to extract information on prompt neutron emission. This will be discussed in the next subsection. 

Fission fragments (pre-neutron emission). The yield curves discussed above refer to fission product yields after emission of prompt neutrons. As discussed above, the physical methods based on momentum conservation at scission (double energy, double velocity, or energy and velocity measurements) allow the measurement of the yield distribution of fission fragments (prior to prompt neutron emission). In these cases, simultaneous information is obtained on the kinetic energy of the fragments detected. 

There is, however, an interesting exception in the figure, namely Md, which shows a neutron yield of about 2 rather than an extrapolated value of 4. The nucleus of Md is the only nucleus among those shown in Fig. 4.20 with a symmetric yield distribution as seen in O Fig. 4.16. (The fermium isotopes with A = 254, 256, and 257 (N = 154, 156, and 157) shown also in Fig. 4.20 have an asymmetric mass distribution (see Fig. 4.16)). Unfortunately, no values of prompt neutron emission are known for the other isotopes with Z > 100 that show a symmetric mass distribution ( Fig. 4.16). 

Cumulative plot of the mass yields for thermal-neutron-induced fission of obtained by summing the yields from very asymmetric fission to symmetric fission for fission products (+) and fission fragments ( ). The numbers of prompt neutrons emitted [vi and for light and heavy fragments, respectively, can be obtained from the horizontal distances between the curves using slight corrections for curvature from (Terrell 1962)... 

So far, essentially mass-yield curves were dealt with. Each point of such a curve represents the formation cross section of isobaric nuclei of mass number A, composed of different combinations of protons and neutrons. Because heavy, fissile nuclei are generally more neutron rich than stable nuclides with about half their mass, fission products are generally also more neutron rich than stable nuclides of the same mass, even after the loss of a few prompt neutrons. (Example The symmetric fission of the compound nucleus (Z - 92, N -144) would form two/raiment nuclei of Pd (Z = 46, iV= 72). Assuming the emission of one prompt neutron, the corresponding primary fission product would be Pd. The stable isobar in mass chain with A = 117 is, however, Sn (Z -50, N- 67). As a consequence, the nucleus Pd would have to undergo a sequence of four P decays to reach stability.) Thus, the products... 

It is interesting to note that the spectra of ternary particles extend to different atomic numbers, which coincide nearly with the size of the neck that results from the postulate extracted from the mass yield and nuclear charge distributions. To correlate the data, it has been postulated that the two spheres of the dumbbell configuration shown in O Fig. 4.11 for the fission of are practically the same for all asymmetrically fissioning nuclei from Z= 90 to about 99 and, consequently, that the variation in the neutron/proton numbers of the different compound nuclei must be connected with the size of the neck. O Fig. 4.29 is the direct experimental proof for this assumption in the fission of uranium, the neck size is (92 — 82 =) 10 protons in the fission of californium, the neck size is (98 — 82 =) 16 protons. The situation is similar for neutrons and for the total mass. This is, however, less convincing due to prompt neutron emission. 

Zoom calculations relating to the four bare systems yielded a functional relation between Keff and exp. This function, a(Keff) was employed in the determination of a(predicted) for the composite systems. The decay constant so determined by reference to the bare systems is a simple function of the prompt neutron lifetime, 1. The eighth column of Table I lists the values of a(predicte<9. However, a small adjustment in a(K ) must be made because Of the slightly different values of 1 in the reflected systems compared to the bare system. In the ninth column, a(predlcted) is adjusted according to the ratio of the values of 1 which were determined with the Zoom code. 

The large lattice pitch and the long neutron diffusion time in the HWR lattice yield a very long prompt-neutron lifetime F, approximately 1 ms in a CANDU reactor, which slows the evolution of power in transients, especially near-prompt-critical power excursions. 

Capture of neutrons in fissionable material, fission, fission products and yield, delayed and prompt neutrons energy release from fission conversions MWD/gm, fissions/watt sec., etc. 

Substituting the bromine-67 half live of 55.7E seconds yields a limiting negative period, T = -80 seconds. No matter how much negative reactivity is added to a reactor, after the initial reduction in prompt neutrons and the decay of short lived fission products, core neutron population will decrease with a -80 second period. A typical power history following a scram is illustrated in Figure 4.3,... 

This experiment is designed to determine the reactivity worth of the control rods by a pulsed neutron technique. A burst of neutrons is injected into the reactor, and the decay rate of the resultant neutron flux is measured. The decay rate measured is that of the prompt fission neutrons and is proportional to the prompt critical reactivity of the reactor. Measurements will be made with the reactor in subcritical conditions and at delayed critical. The decay rate at delayed critical yields the constant of proportionality between the decay rate of the neutron flux and the reactivity in dollars. This constant is equal to the ratio of the effective delayed-neutron fraction to the prompt-neutron lifetime. 

Since a nuclear reactor is a statistical system, it will show fluctuations in neutron intensity. These fluctuations, or pile noise, are not commonly considered of interest in themselves, but only as interference to other experiments. However, since the nature of the pile noise depends strongly on important reactor parameters, its study can enable the determination of quantities less easily accessible by other means. In particular, Moore (f) points out that the noise spectrum of such a system, that is, the mean square noise amplitude per unit band width, is proportional to the square modulus of the transfer function or to the Fourier cosine transform of the autocorrelation function. Thus, observation of the noise spectrum of a reactor could yield information about the shape of its transfer function. To test this technique, pile noise analyses were done on various low-power experimental reactors at Argonne National J aboratory. Since these reactors operate at such a low level that power effects on reactivity do not appear, the shape of the low-frequency portions of their transfer functions would depend only on fairly well-known delayed neutron parameters, and thus would be of little interest. However, the high-frequency rolloff portion of the transfer function is strongly dependent on the quotient of the effective delayed neutron fraction over the prompt neutron... 

If the spin of the initial state is very different from that of the capturing state, we should expect to find that the intensity of a low energy y-ray depends markedly on which of the two spin states is responsible for capture. For prompt y-rays this phenomenon has not yet been studied, but the corresponding effect on the yield of isomeric states has been noted by several authors. It seems to have been first detected by Capron and Verhoeve-Stokkink. In more detail it has been studied by Sailor, who has examined the activation of the isomeric state of In produced by neutron capture in the first two resonances in targets of In, and by Wood in studies of the activation of the isomeric state of Eu 2... 

For example, consider the Nephelauxetie Effeet. Analysis of the interelectron repulsion parameters derived from analyzing the d-d speetrum invariably leads to lower values than in the free ion. The interpretation is that, in the eomplex, the d eleetrons are, on average, further apart which is consistent with expanded rf-funetions in the eomplex and/or with d electron delocalization onto the ligands. Analysis of the electron density distribution from X-ray diffraction in trans-[Ni(NH3)4(N02)2] yields a rf-orbital radius larger than that for free Ni. However, the unpaired electron density derived from polarized neutron diffraction (PND) data yields a if-orbital radius less than for free Ni " prompting Figgis to propose an anti-Nephelauxetic effect. DFT calculations support LFT in that the d-orbitals expand upon complex formation but also provide an explanation of the diffraction data. 

The very basis of PGAA, neutron capture followed by de-excitation of the compound nucleus through emission of characteristic gamma rays, provides information on the nuclear properties of the absorbing nuclide. Careful measurements permit determination of the prompt gamma-ray production cross section, which may be decoupled into the gamma yield, and neutron capture cross section. Quantitative PGAA maybe based on comparison with element standards... 

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