Neutron counters

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

The high cross-section for thermal neutrons results in the use of boron and boron compounds for radiation shielding (14). The ease of detecting the a-particle produced when boron absorbs thermal neutrons results in the use of boron for neutron counters as weU. 

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. 

Time variations in the intensity of the flux during irradiation. This is an important consideration only when a single sample transfer system is used. Gas-filled BF3 neutron counter tubes are often used to monitor the neutron flux in order to normalize the data when the sample and the standard are not irradiated simultaneously. Gain shifts and dead-time effects associated with the use of neutron monitoring detectors also contribute to the errors associated with a single sample transfer system. 

Fig. 6. Large neutron counter with 3He counting tubes for the detection of neutron bursts emitted in the spontaneous fission of superheavy nuclei. Reproduced from R.L. Macklin et al. [43], Copyright (2002), with permission from Elsevier Science.

CuO heated in a furnace to oxidize CO to CO2, Ascarlte again to. remove this CO2, Cu or Mg heated in a furnace to remove Oa and finally through active charcoal at liquid air temp, to remove He. The gas that comes through is F2 and is measured by a neutron counter. 

After leaving these traps, the air stream passed through about 10 ml CCI4, where the Br activity remained and from there through some traps to a vacuum pump, and on to the exhaust vent of the room. A neutron counter is situated adjacent to the CCI4 solution. 

Die soln. Is then added to a separatory funnel containing 50 ml CCI4 and 25 ml 6 1 HHOa- Ihus, when the bo In. becomes acid, Bra Is liberated and extracted In the CCI4. Whereas very little Iodine Is extracted since It Is present as IOg. (Hand shaking Is hotter, rather than mechanical.) The CCI4 phase Is measured by neutron counters, field - 1A-U20. 

The concentration of the U-nltrate soln. Is held down bo that the CClt would settle promptly after the funnel has been shaken. 1 ml of cone. SI le present In the eqln., and after Irradiation 10 ml of 5f BaBOjg soln. are added. Following shaking and settling, the CCI4 layer le drawn off end measured with a neutron counter. 

The operation principle of pulse channel for measuring the coimt rate is based on the neutron registration by the detector of ionization fission chamber or neutron counter, pulse transmission from the detector via the communication lines to the preamplifier inlet and their following amplification, formation and processing by means of the auxiliary electronic equipment. 

Use Neutron counter, radiation shielding (in the form of boral), medicine. 

Neutron counters located close to a reactor core are subjected to both neutron and gamma bombardment. Although a neutron counter—e.g., a °B counter—is mainly sensitive to neutrons, it responds to gammas too. At low reactor power, when the neutron flux is small, the neutron signal is overshadowed by a signal due to gammas emitted from fission products that had been accumulated from earlier reactor operation. To eliminate the effect of the gammas, a compensated ion chamber is used. 

Show that the sensitivity of a neutron counter (BF3 or boron-lined or fission counter) decreases with time as exp( —o- <)>t). 

Thresholds for slow neutron production in dn) reactions have been observed by Bonner and Cook the ratio of the counting rate in a slow neutron detector to that in a fast neutron counter increases sharply when the incident deuteron energy becomes sufficient to excite a new level of the residual nucleus [see for example Be [dn) Sect. 50]. 

Large liquid scintillation neutron counter with pulse height analysis. Attenuation measurement... 

Fig 29. The number of neutron-proton coincidence as a function of neutron angle from the photodisintegration of deuterium by 280 Mev photons. In these measurements the proton counter was kept fixed at 76 (on the side opposite to the neutron counter) and the energy of the protons accepted by the detector was 129 Mev. The width of the curve arises from the finite angular resolution of the equipment. [Figure from Wattenberg et aU (to be published).]... 

In PHENIX and SUPERPHENIX, primary sodium is taken directly from the vessel and transported by an assembly of pipes and pumps to a module located on the reactor slab. The presence of any delayed neutron emitters is then revealed by neutron counters. 

The handheld neutron monitor (HHNM) is a portable ( 4 kg) neutron detection device with three He proportional neutron counters, a GM counter and integrated electronics, which provide a means of searching for and localizing neutron radiation sources. A measurement sequence consists of background and verification measurements. When a predetermined threshold is exceeded, the detector triggers an alarm and records the relevant information. 

The He neutron counters detect all neutrons arising from both (a, n) and fission reactions. Sophisticated pulse processing electronics, called neutron coincidence counting circuits (shift register), measure the number of neutrons that are detected within a predefined time interval (gate width), and differentiate between time correlated (coincidence) neutrons emitted from the fission events and single neutrons created as a result of a-particle interactions. The measured coincident neutrons (doubles) are proportional to the mass of the even-even Pu isotopes ( PUefFecdve = 2.43 Pu -I- Pu -I- 1.69 Pu). 

The drawer neutron counter (DRNC) has been designed to perform the assay of plutonium in fast critical assembly fuel drawers (Krick and Menlove 1980). Eight tubes (2.5 cm diameter by 91 cm active length) were used in the system. The principal feature of the neutron coincidence detector is a 7 cm by 7 cm by 97 cm detector channel, which provides a uniform neutron detection efficiency of 16% along the central 40 cm of the channel. 

The spent fuel neutron counter (SFNC) is a prototype neutron-detector system that verifies closely packed spent fuel assemblies stored in a spent fuel pond (Ham et al. 2002). The system contains a fission chamber moderated by a polyethylene cylinder housed in a watertight stainless steel enclosure. The SFNC measures total neutron signals from long-cooled spent fuel assemblies while in their storage position, without requiring them to be moved. The technique can detect a missing fuel assembly. These measurements are performed underwater in a gap between four assemblies. 

The Direct Use of PWR spent fuel in CANDU (DUPIC) safeguards neutron counter (DSNC) is a well-type neutron coincidence counter with 18 He detectors configured with appropriate shielding, which measures all types of highly active materials from the dry reprocessing process for CANDU bundles (Menlove et al. 1997). It derives plutonium and uranium contents from the measured contents. Plutonium- and uranium-curium ratios... 

Similar calculations were performed for 97% enriched uranium scrap in 4.75- and 6.00-in.-dlam bottles placed in the active neutron counter. The smaller diameter container was found to be considerably less reactive and hence had a much less restrictive wei t limit than the larger bottle. 

Nondestructive-type instrumentation is used to assay each scrap package before it enters the process, and Nal single-channel analyzers are used to note inventory buildups by measurement of the 414 keV of Pn. Also, continuous specific gravity and nitric acid concentration instruments help assure low plutonium losses to the waste system. Both gamma and neutron counting num-itors are used to measure plutonium content of waste streams and waste tanks. Figure 1 shows a simplified sketch of the PRF and its instrumentation for criticality prevention. Figure 2 shows how inventory buildup within the reflux solvent extraction system is monitored by neutron counters. ... 

R. H. STEVENS and R. C. SMITH, "A Portable Neutron Counter for Determining Weights of Deposits of Uranium-Fluorine Compounds, K-1586, Oak Ridge Gaseous Diffusion Plant (1964). 

Using both calibrated neutron counters (activated Ag — GM tubes) and NEPy neutron fluence anisotropies have been measured. The NEP are distributed as for spectral measurements while the GM counters are placed farther away 2.5 m). A clear anisotropy cut off when po is increased at constant Vo is shown in Fig. 3. Anisotropy measurements carried out with NEP and GM tubes are in excellent agreement with each other, see Fig. 8. There exists no correlation between anisotropy and neutron yield. Anisotropy decreases with an increase of the level of stored energy and the standard fluctuations shot to shot are very small. Table I summarizes the results. 

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