Beta-delayed neutrons

Exotic Nuclei and Their Decay. As reported by J.C. Hardy (Chalk River Nuclear Laboratories. Atomic Energy of Canada, Ltd.), recent advances in nuclear accelerators and experimental techniques have led to an increasing ability to synthesize new isotopes. As isotopes are produced with more and more extreme combinations of neutrons and protons in their nuclei, new phenomena are observed, and the versatility of the nucleus is increased as a laboratory for studying fundamental forces. Hardy reports that, among the newly discovered decay modes are (1) proton radioactivity, (2) triton, two-proton, two-neutron, and three-neutron decays that are beta-delayed, and (3) 14C emission m radioactive decay, Precise tests of the properties of the weak force have also been achieved. 

The determination of Pn values is based on the beta saturation counting rate (cP ), the neutron saturation counting rate (C11 ), the beta-neutron coincidence saturation counting rate (dP ), the beta counting efficiency (eg), and the neutron counting efficiency (en). The usual relation for the delayed neutron emission probability is... 

The fluctuations in neutron peak intensities arise from the Porter-Thomas distributed beta decay widths to levels in the NE nuclide. In the simplest case only a single state in the GC nuclide can be fed and only one neutron partial wave is significant. The observed levels will be a subset of levels in the NE nuclide and will be distributed in energy following a Wigner distribution. In a typical GC nuclide, however, there will be a number of accessible final states and the delayed neutron spectrum will be a superposition of transitions from several parts of the NE nuclide level structure. 

The prompt neutrons emitted in fission are available for fission in other nuclei - hence the chain reaction. The fission fragments formed initially are rich in neutrons. For example the heaviest stable isotopes of krypton and barium are 86Kr and 138Ba. Excess neutrons are emitted from the fission fragments as delayed neutrons or converted to protons by beta decays. For example... 

Ttwo DN monitoring systems are located adjacent to the primary cooling loops to detect the delayed neutrons emitted from precursors released into the coolant sodium. The CG precipitating system detects fission product of Rb i.e. beta decay of Kr released into the cover gas argon. 

A number of uncommon decay modes exist which are of little direct relevance to gamma spectrometrists and I will content myself with just listing them delayed neutron emission, delayed proton emission, double beta decay (the simultaneous emission of two 3 particles), two proton decay and the emission of heavy ions or clusters , such as and Ne. Some detail can be found in the more recent general texts in the Further Reading section, such as the one by Ehmann and Vance (1991). 

DELAYED NEUTRON A neutron emitted by a fission product in an isobaric chain the delay caused by the time taken for beta decay in the chain. 

Fission Fragments Prompt Neutrons Prompt Gamma Rays Delayed Neutrons Isomeric Gamma Rays Fission Product Qeunmas Fission Product Betas Fission Product Anti-Neutrinos... 

The presence of the delayed neutrons is not very important to the steady-state operation of a stationary-fuel reactor. In such a reactor at any time instant the beta emitters are located essentially at the point of the original fissions, and these nuclei are present in concentrations which reflect the rate of fissioning at some previous time. However, since the flux and fuel concentrations are constant in time (over times of the order of 100 sec or longer), the number of delayed neutrons being produced per unit time at any instant is given by... 

Delayed neutron precursors decay by beta decay. Which ONE reaction below is an example of beta decay ... 

From Table 3,3, the precursor with the longest half-life is bromine-87 the half life is 55.7 seconds. Bromine-87 is significant because when the reactor is shutdown and prompt and shorter lived delayed neutrons have decayed away, the core neutron population is maintained by delayed neutrons produced by the decay of bromine-87. The decay scheme for bromine-87 is shown in Figure 3,2. Approximately 30 percent of bromine-87 nuclei decay by beta emission to krypton-87 at ground state energy while 70 percent decay by beta emission to an excited state of krypton-87. Of the later 70 percent, about two percent of the excited krypton-87s... 

The residence time was determined for our neutron counter by measuring the time intervals between beta start signals and neutron stop signals. With a residence half-time of 11 ms and a coincidence resolving time of 40 ms. 92 of the true coincidence events were included. The fraction of true events not detected does not influence the present results because we normalize the Pn measurements to a known Pn value measured under identical conditions. The coincidence rate was measured by a simple overlap coincidence module where the beta pulse Input was stretched to 40 ms by a gate and delay generator. To measure the accidental coincidence rate, the same beta pulse was sent to a second coincidence module and overlapped with neutron pulses which had been delayed 45 ms. After correcting each coincidence rate for deadtime effects, the difference was the true coincidence rate. 

FIGURE 32-6 Overview of the neutron activation process. The incident neutron is captured by the target nucleus to produce an excited compound nucleus, which de-excites with emission of a prompt gamma ray. The radioactive nucleus formed decays by emitting a beta particle. If an excited product nucleus is formed, a delayed gamma ray can be emitted. If decay is directly to the ground state of the product nucleus, no gamma ray is emitted. 

Highlights. Treatment of the radioactive waste from nuclear reactors is one of the points that receive wide public attention and the disposal and burial of HLW in particular is a contentious issue due to the concerns about leakage to the environment. The technical solutions that are currently used to treat the waste that were listed earlier (concentrate-and-contain, dilute-and-disperse, and delay-and-decay) are not suitable for HLW, where safer solutions like vitrification or Synroc are sought. The characterization of the LLW and MLW waste is not as complicated as that of spent fuel but stiU greatly more complex than analysis of fresh fuel. Some of the procedures and methods used in other parts of the NFC are suitable for LLW and MLW. The composition of HLW must be determined in order to estimate the decay rate of the radioactivity and to classify the required protective measures that depend on the radionuclides and their products (emitters of alpha, beta, gamma, and neutrons). 

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