Thermal-neutron column

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... 

Some research reactors are equipped with a so-called thermal column of about 1 m X 1 m X 1 m, consisting of blocks of graphite and installed near the reactor core. Due to the moderator properties of graphite, only thermal neutrons are present in such a column. 

The more important of these reactions will be considered below. Reactions 3, and 4 are brought about by the fast neutron flux in the pile, which flux Mellish et al. 59) calculated to be 0.17 of the slow flux in the center of the Harwell pile (BEPO). Fortunately, however, the cross sections for these reactions are usually considerably lower than those for normal (TO,y) reactions induced by thermal neutrons. Contributions brought about by n,p) and (to, ) reactions can often be greatly reduced, as mentioned previously, by irradiating in the thermal column of the reactor, with some loss of sensitivity. Reaction 4, in,2n), is produced by... 

A sample of an iron meteorite was irradiated with thermal neutrons, dissolved in concentrated HC1 in the presence of H2O2, evaporated to drynes, redissolved and submitted to SPE with a polyaniline column. Pt, Pd and Au were eluted from the resin with aqueous thiourea and Ir with aqueous ascorbic acid and measured. Polyaniline resin has great affinity for Pt(IV), Pd(II), Ir(IV) and Au(III) ions in 0.1 M HC1 as the acid concentration rises up to 10 M, only Au(III) remains strongly attached to the column157. 

Fission cross sections are denoted by For fissionable isotopes of thorium and elements of higher atomic number, the average number of neutrons produced per fission is listed in the same row as the fission cross section, in the same column as the mass, to conserve space in the table. The average number of prompt and delayed neutrons produced by fission with a thermal neutron is denoted by V. The average number of prompt neutrons produced by fission with a thermal neutron is denoted by Vp. The average number of neutrons emitted per spontaneous fission is denoted by t jp. ... 

Neutron Curtain in the HTR. A 6 ft 6 in. square sheet of -in.-thick boral mounted in an aluminum frame will serve as the neutron curtain. This curtain will be used during operation, if desired, to cover the face of the thermal column and reduce the number of thermal neutrons entering the column. It will have rollers at its four corners that will travel in vertical guides mounted in the 3-in. space between the two 4-in. thermal shield plates. 

The holes normal to the direction of the thermal column are expected to be extremely useful in providing very pure fields of thermal neutrons although at somewhat reduced intensity. 

The fission chamber was first placed in a graphite column containing an Sh-Be source at a position where the thermal-neutron flux and t he Cd ratio were known from standard indium foil measurements. By exposing the chamber... 

The third column contains branching ratios of 7-transitions after thermal neutron capture [95Ge06],... 

The most practical neutron source for NAA is a nuclear reactor, which produces neutrons via the nuclear fission process (see Chap. 57 in Vol. 6). Many research reactors are equipped with irradiation facilities that provide a stable, well-tailored, isotropic neutron field with sufficiently high flux. Low-energy (thermal) neutrons comprise the most important part of the reactor spectrum hence the degree of moderation is an important parameter. The irradiation channels are usually created in moderator layers, such as a thermal column or a Be reflector blanket. 

Fermi chopper thermal neutron time-of-flight experiments using a thermal column. 

Total fluence requirements are determined by the U concentration present in the mineral to be examined. Apatites typically have concentrations from 5 to 100 ppm, requiring a thermal neutron fluence of about 10 n cm. Zircons, with 50 to 500 ppm, require around 2 to 4 x 10 n cm. As a practical example, this translates to about 30 hours in the thermal column of a 1 MW reactor. However, the irradiations do not have to be continuous. 

Highly thermalized neutrons are required such as those produced in a thermal column. For irradiation, mineral grains are mounted, polished and etched, then sandwiched between thin... 

Solid 002(0211302)2 (natural uranium) was Irradiated with thermal neutrons for 10 min (flux not given) and the target was dissolved In concentrated hydrochloric acid. The solution was boiled for several minutes and was then made. 2-3 molar In the acid. A copper foil was introduced Into the solution and the fission product tellurium plated out on the foil. The coated foil was dissolved In 1 M nitric acid and passed through a cation exchange resin column (resin not designated) to remove the copper. The eluate was made basic with ammonia water. Iodide carrier added, and permitted to grow In for 70 m1n. The Iodide was oxidized to the free element and a number of I2 - I redox cycles were carried out. The Iodine was finally precipitated as the silver salt. 

This NAA technique is based on the nuclear reactions 23Na(n,7)24Na and 41K(n/y)42K. Half-lives of the activated products are 15.0 hrs and 12.4 hrs, respectively. For Na analysis, the samples were irradiated in a specially designed thermal column to suppress the fast neutron reaction of 27Al(n,a)24Na which interferes with the reaction for Na. For K analysis, the proplnt samples were irradiated at a standard irradiation position of the reactor. For the Na irradiations, the neutron flux in the thermal column was in the order of 1010, whereas for the K assays it was approx 1012 neutrons/cm2-sec... 

Wiggins et al. [456] used neutrons from the thermal column of a 10 kW pool-type research reactor and from a 120 pg Cf source to study the prompt photon emission resulting from neutron capture in magnesium nodules (ter-romanganese oxides) from the ocean floor. Spectra were recorded with a Ce(Ii) detector and a 1024-channel analyser. Complex spectra were obtained by irradiation of seawater, but it was possible to detect and estimate manganese in nodules in a simulated marine environment by means of the peaks at 7.00, 6.55, 6.22, and 6.04 pV. 

Neutron diffraction patterns of powder samples were taken on a neutron diffractometer (X = 1.085 A) mounted on the thermal column of a WR-SM nuclear reactor [3]. The DBW-3.2 program for the Rietveld neutron diffraction line shape analysis was used in calculations and structure refinement [4]. A DRON-3M X-ray diffractometer (CuK - radiation) was used to measure X-ray powder diffraction patterns. 

Self-shielding of resonance neutrons can be minimized by irradiating in the graphite-loaded column (thermal column) of the pile. Here the contribution by neutrons of energies greater than thermal is considerably reduced. 

The determination of sodium in pure aluminum by the reaction Na (7i,y)Na is complicated by the reaction Al (n, )Na brought about by the fast component of the neutron flux spectrum. Salmon (77) determined the contribution to the sodium content by this reaction in BEPO by irradiating identical samples and standards normally and in the thermal column. The difference in the sodium content determined by these two methods gives the apparent sodium content derived from the (n.,o) reaction. Salmon found this value to be 81 ppm under the conditions of the experiment. 

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