Neutron absorbers 454 Subject

We have shown the safety rod 176 as being suspended over the upper maximum level of the slurry in the reaction tank 1 and within the space between the cadmium plates 161—162 and the top of the reaction tank. In this region the safety rod 176 is subjected to only a low density of slow neutrons to which the safety rod is very absorbent. Neutron absorbers inserted into the slurry are subjected to high neutron densities and they cannot continue to absorb neutrons indefinitely. The continued absorption of neutrons by the absorbing material causes transmutation of the absorbing materials and an element or isotope may be built up within the material which has a smaller neutron capture cross-section than the original material. However, by maintaining the safety rod within the space above the cadmium plates 161—162, this reduction in efficiency of neutron absorption Is reduced to a minimum. Consequently, unless the safety... 

Anoher object of the invention is to provide a simple and sensitive means for controlling the neutronic reaction while at the same time producing a useful material by subjecting neutron absorbent material to intense neutron bombardment. This object is accomplished by dividing the reactor into a plurality of sections or cells, the central group of which comprises reactive composition between spaced portions of fertile material, and the outer group of cells comprises fertile material only. Thus a reactive zone is provided which is completely surrounded by an absorption zone of the particular fertile... 

These data show that the K-effective from any one calculation may vary from the actual value by a significant amount, b fact, another paper on a similar subject also illustrates how this pitfall can occur when the fission density distribution does not adequately account for the distribution of fissile material. The system discussed here illustrates a situation where the fission density distribution is strongly influenced by the presence of a lumped neutron poison. This can also contribute to uncertainties in the calculated K-effectives. The user can most easily recognize the potential for errors of this kind by comparing the relative fission densities in localized sections of the system with the fissile material distribution. The optimum condition would occur when the two distributions are the same, hi fact, this should be considered any time the fissile material is distributed in an unsymmetrlc or complex manner. This also must be considered any time there are lumped-neutron absorbers in the system. 

Complexes of monoamide derivatives of dtpa with Ln + ions have been studied. Their stability constants are, as expected, less than those of dtpa itself Using [Gd(DTPA)] it has now been shown that these complexes get absorbed by the DNA of the cell they have been used to locate. If the gadolinium is subjected to thermal neutron treatment, short-range high-energy electrons are emitted that can kill the tumour cell whilst nearby healthy tissue is unaffected. ... 

Baryonic candidates were traditionally brown dwarfs, old white dwarfs, neutron stars, and stellar mass black holes. Both they and the new ones are subject to the big bang nucleosynthesis constraint above others might accrete matter, thus radiating, absorb at some wavelengths, or otherwise reveal themselves. 

For each absorbed x-ray photon or neutron, the proportional or scintillation counter produces a discrete electric pulse. The flux J of the beam of x-rays or neutrons is measured as the number of counts of such pulses observed per second. If measurements are made repeatedly with a beam of constant flux, the number of counts observed during a fixed time period is not exactly the same, but is rather subject to statistical fluctuations. The arrival time of any one particle (x-ray photon or neutron) is totally uncorrelated with the arrival time of the next particle. The flux J of the particles,... 

A common source of error arises when sample and standard are not subjected to the same neutron flux as a cause of horizontal or vertical flux gradients. For accurate analyses, they have to be compensated or eliminated (e.g., by rotating the samples). Selfshielding may also introduce significant errors, namely when there is a difference in absorption by the matrices of sample and standard. In general, biological matrices are poor absorbers of neutrons however, the phenomenon must be kept in mind when designing the standard. 

When used in place of hydrogen, deuterium or (sometimes designated as D) results in water approximately 10 percent denser than normal. Termed "heavy water," D O is harmless in small doses and can therefore be used safely as a tracer in the body, most commonly in measuring a subject s metabolic rate. Heavy water is also used as a neutron moderator, meaning it is able to slow neutrons by collisions without absorbing them.This process is crucial for the chain reaction in nuclear reactors, where fast neutrons are produced by the fission process, but slow or thermal neutrons are more likely to induce fission. 

The Laboratory s Criticality Safety Committee subjected the absorber to ad tional tests to determine its other proper ties. To determine whether the absorber lost its neutron-... 

There is a very small amount of tritium (H ) produced in pure water either from one hydrogen capturing two neutrons or from one neutron being absorbed in the one atom of deuterium (H ) per over 7 0 00 hydrogen atoms in normal water. The amount is so small as to be not worth mentioning (2-3 Bq/gm), but while on the subject of activity produced by water, there are two other things you should remember. 

The specimens (absorbers) consisted of 25-40 pm thick foils. Due to the higher neutron embrittlement and ageing sensitivity of VVER 440 (V-230) nuclear reactors, our study focused on the Russian 15Kh2MFA steel used in this older VVER-440 reactor type. The samples were subjected for one, two and three years to a neutron fluence ranging from 7.8 x 10 m to 2.5 x 10 " m . Taking into account the fact that the irradiation of the specimens was enhanced by their close proximity to the reactor core (leading to an accelerating factor of about 10), radiation treatment of the specimens after three years was equivalent to about 30 years in the real pressure vessel (the projected lifetime of VVER-440 V-230 reactors). More about this study can be found in [194],... 

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