Fission neutron emission

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Some materials have a spontaneous decay process that emits neutrons. Some shortlived fission products are in this class and are responsible for the delayed neutron emission from fission events. Another material in this class is Cf that has a spontaneous fission decay mode. Cf is probably the most useful material to use as a source of neutrons with a broad energy spectrum. 

Cf spontaneous fissions have a fast neutron energy spectrum, shown in Figure 3, with an average energy of 2.2MeV. On average, 3.76 neutrons are emitted per spontaneous fission. The neutron emission rate is 2.34 X 10 n/(s-g)... 

Nuclear detection approaches that use radioactive isotojjic sources (e.g., Cf for spontaneous fission and asociated neutron emission or ° Co for gamma emission) will have to obtain state and federal hcenses to field the equipment and abide by apphcable health and safety regulations. The Hcensing process takes some time to put into place and may restrict the easy movement of the detection equipment to new locations. This impacts the abffity to rapidly re-locate equipment based up inteUigence estimates of the behavior of smugglers. The use of fixed pre-licensed sites can help to some extent. 

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). 

If the time scale of neutron capture reactions is very much less than 3 -decay lifetimes, then rapid neutron capture or the r process occurs. For r-process nucleosynthesis, one needs large neutron densities, 1028/m3, which lead to capture times of the order of fractions of a second. The astrophysical environment where such processes can occur is now thought to be in supernovas. In the r process, a large number of sequential captures will occur until the process is terminated by neutron emission or, in the case of the heavy elements, fission or (3-delayed fission. The lighter seed nuclei capture neutrons until they reach the point where (3 -decay lifetimes have... 

We discuss RPA calculations of the Gamow-Teller properties of neutron-rich nuclei to study the effect of B"delayed fission and neutron emission on the production of Th, U and Pu chronometric nuclei in the astrophysical r-process. We find significant differences in the amount of -delayed fission when compared with the recent calculations of Thielemann et al. (1983). 

KRU81]). The B strength function for nuclei along the decay back paths [coupled with neutron separation energies (Sn), fission barrier heights (Bf) and B"decay Q-values (Qg)] determines the amount of B delayed fission and neutron emission that occurs during the cascade back to the B stability line. 

Californium Assay dnd Analyses. Quality control for the californium feedstock is accomplished by measuring the neutron emission rate of an aliquot of the starting material and by performing analyses for isotopic content and chemical purity. Neutron emission rate is measured in a fission counter (9). Isotopic content is measured by mass spectrometry and chemical purity by spark source mass spectrometry. The completed assembly is leak tested, decontaminated, and assayed before packaging and shipping. 

Neutron emission immediately following p transmutation ( ff -delayed neutron emission) is observed for many neutron-rich nuclides, such as Br and many fission products. Delayed neutron emission is very important for the operation of nuclear reactors (chapter 10). 

Fission of heavy nuclei always results in a high neutron excess of the hssion products, because the neutron-to-proton ratio in heavy nuclides is much larger than in stable nuclides of about half the atomic number, as already explained for spontaneous hssion (Fig. 5.15). The primary fission products formed in about 10 " s by fission and emission of prompt neutrons and y rays decay by a series of successive / transmutations into isobars of increasing atomic number Z. The final products of these decay chains are stable nuclides. 

When nuclei are very proton-deficient or very neutron-deficient, an excess particle may boil off, that is, be ejected directly from the nucleus. These decay modes are called neutron emission and proton emission, respectively, and move nuclides down or to the left in Figure 19.1. Finally, certain unstable nuclei undergo spontaneous fission, in which they split into two nuclei of roughly equal size. Nuclear fission will be discussed in more detail in Section 19.5. 

If the nuclear flow towards increasing Z values reaches the actinide or transactinide region, it is stopped by neutron-induced or /3-delayed fissions which lead to a recycling of a portion of the material to lower Z values. At freezing of the neutron captures or inverse photodisintegrations, mainly /3-decays, but also spontaneous or /3-delayed fissions and single or multiple /3-delayed neutron emissions, drive the neutron-rich matter towards the valley of stability. These post-freezing transformations are shown schematically in Fig. 22. 

Figure 3.19 The fission fragments FFi and FFj from fission are neutron-rich. They reduce their neutron number either by beta decay or by neutron emission.

Decay modes are a = alpha particle emission (B = negative beta emission p+ = positron emission EC = orbital electron capture IT = isomeric transition from upper to lower isomeric state n = neutron emission sf = spontaneous fission (B(B = double beta decay. Total disintegration energy in MeV units. 

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