Pulsed neutron irradiation

High energy neutron generators are particularly useful in well logging applications. In such applications one important factor is accurate knowledge of the neutron pulses that irradiate the surrounding formation. For example, it is desirable to accurately measure the neutron output, e.g., the number of neutrons emitted by the neutron detector. 

In our lab, R.T.Ds are experimentally determined for the 3 phases by pulse injections of radioactive tracers in the real medium gaseous Ar or Kr liquid organometallic complexes, or halides solid neutron-irradiated metal catalysts. 

Pulsed Reactor Irradiation In the KIWI—TNT test of the NERVA program, BlkPdr was the only igniter tested in a pulse environment (Ref 192). In a M msec pulse, the max dose the BlkPdr was exposed to was 3.0 x 1014 n/cm2 fast neutrons, 1.1 x 101S n/cm2 thermal neutrons and 1.5 x 107 R gamma. On the various tests conducted no adverse effects were evident... 

A laboratory technique that is commonly used to calibrate particle velocity is the tracer technique. The tracers can be optically illuminating particles or short-lived radioactive tracers. We will briefly discuss two techniques that can be used to produce radioactive tracers on-line pulsed neutron activation (PNA) and off-line irradiation. 

Pulsed-neutron source data, such as shown in Fig. 1, have been obtained at Ouiford and analyzed as described above on a miniber ai different systems. These measurements have been made on ea rimental assemblies about 100, and less, sUbcritical, and on plant systems down to about 50 subcritical. They have included e q)erimental assemblies of PuO and PuQi-llOi mixtures, low enriched urajqrl nitrate, a large underground waste crib, and irradiated ffiel elements. Measurements on these systems and others will be described and discussed. 

The reactivity of the fuel loadings was monitored with three detectors one proportional counter and two fission chambers. For spent-fuel measurements, three fission chambers were used. Approach-lo- critical and pulse-neutron source data were recorded during loading and unloading of fuel tubes. A k-eff value of 0.94 was obtained with a loading of 54 tubes of unirradiated fuel, compared with a value of 0.95 obtained with 91 tubes of fuel irradiated to an average exposure of 3020 MWd/MT- The loadings were limited to a maximum k-eff value of 0.97 established as the test safety iimit. ... 

In Chapter 3, Section 3.6.7, I explained that fast neutron irradiation creates a uniform field of traps throughout the detector. This being so, the resolution enhancer can correct for this additional trapping in the same way. However, neutron damage in n-type detectors creates hole traps and it turns out that because these are the minority carriers the method will not work. Even so, this would have been a useful device to have available for p-type damaged detectors which are less successfully repaired by annealing. The manufacturer s literature quoted improvements of more than a factor of two in resolution, for example, from 4.26 to 2.02 keV in a particular case. Unfortunately, these modules seem to have been withdrawn from the market This may be because digital pulse processors are able to perform the same sort of function as part of their normal operation. 

A typical procedure for Na assay is One gram proplnt samples sealed in polyethylene vials are irradiated for at least seven hrs in the thermal column. In order to minimize any thermal neutron flux gradient, the samples are rotated uniaxially at 60 rpm. Following irradiation, the samples are allowed to decay for approx 15 hrs to permit all short-lived radioisotopes to decay to insignificance. The 1.369 MeV 7-photopeak of each sample is then counted for 10 min with a 7.6cm x 7.6cm Na iodide scintillation detector coupled to a 400-channel pulse-height analyzer... 

The NAA measurements on the paper samples were made at the Breazeale Nuclear Reactor Facility at the Pennsylvania State University with a TRIGA Mark III reactor at a flux of about 1013 n/cm2-sec. Samples were irradiated from 2 to 20 min and counted for 2000 sec, after a 90 min decay time for Ba and a 60 hr decay for Sb, Analyses were performed instrumentally, without radiochemical separation, using a 35cm3 coaxial Ge-Li detector and a 4096-channel pulse height analyzer. With these procedures, detection limits for Ba and Sb were 0.02ug and 0.001 ug, respectively. These sensitivities are comparable to those obtained by GA s radiochemical separation procedure, and are made possible by the use of the higher neutron output from the more powerful reactor and in combination with the higher resolution solid state detector... 

The neutron activation method for the determination of arsenic and antimony in seawater has been described by Ryabin et al. [66]. After coprecipitation of arsenic acid and antimony in a 100 ml sample of water by adding a solution of ferric iron (10 mg iron per litre) followed by aqueous ammonia to give a pH of 8.4, the precipitate is filtered off and, together with the filter paper, is wrapped in a polyethylene and aluminium foil. It is then irradiated in a silica ampoule in a neutron flux of 1.8 x 1013 neutrons cm-2 s 1 for 1 - 2 h. Two days after irradiation, the y-ray activity at 0.56 MeV is measured with use of a Nal (Tl) spectrometer coupled with a multichannel pulse-height analyser, and compared with that of standards. 

L. Avrami et al, Neutron Pulse Irradiation of Electroexplosive Devices , Defense. Nuclear Agency Rept DNA 4032P, Vol 3, paper 5.4, pp 133-52, Proc 5th Symp, Nuclear Survivability of Propulsion and Ordnance Systems (1975) (SRD) (paper unclassified)... 

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

When short-lived isotopes are involved, a higher activity is produced by irradiating the sample in a reactor that can be pulsed (see Lenihan et al.). Such a reactor producing a high flux of about 10 neutrons/(m s) for a short period of time (milliseconds) is the TRIGA reactor, marketed by General Atomic. 

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