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leila
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Why do gases absorb neutrons less readily than water?
Any ideas on the above question?
Leila
Any ideas on the above question?
Leila
Well that actually depends on the gas. He4 is a very stable nucleus, 2 protons and 2 neutrons in a very stable configuration. So it effectively does not absorb neutrons. On the other hands, He3 interacts non-elastically with neutrons (n,p) with a microscopic cross-section of about 5330 barns. In fact, in certain transient fuel experiments, coils of He3 are used just prior to the test to shield the test rod.leila said:Why do gases absorb neutrons less readily than water?
Any ideas on the above question?
Astronuc said:Each atom has an energy-dependent microscopic cross-section ([itex]\sigma[/itex]) associated with it. The microscopic cross-section is the interaction rate per atom in a target per unit intensity of the incident beam, and in a sense is a probability of interaction. It's unit is the barn and 1 barn = 10-24 cm2.
Multiplying the microscopic by the elements (isotopes) atomic density yields the macroscopic cross-section ([itex]\Sigma[/itex]). The macroscopic cross-section is simply the probability of interaction per unit length of distance traveled by a particle, e.g. a neutron. See example calculation of macroscopic cross-section here - http://www.tpub.com/content/doe/h1019v1/css/h1019v1_117.htm
The density of gases is about 2 or 3 orders of magnitude less than water, which is usually pressurized.
CarlB said:It's not that simple. See fig 1. from the following paper, which shows that liquid H2 and gaseous H2 have very different cross sections, though the cross sections approach one another at high enough neutron energies:
http://ucn.web.psi.ch/papers/PhysRevLett_94_212502.pdf
Note that the above are total cross sections (most of which is scattering), and not absorption cross sections.
By the way, I read the OP's question as having to do with the phase difference for the target, and replaced "water" with "liquid", so the comparison is of one phase with another for the same target molecule. Otherwise the question doesn't make a lot of sense.
Carl
It is that straightforward, although the calculations are not necessarily trivial.Multiplying the microscopic by the elements (isotopes) atomic density yields the macroscopic cross-section ([itex]\Sigma[/itex]). The macroscopic cross-section is simply the probability of interaction per unit length of distance traveled by a particle, e.g. a neutron.
Gases, by definition, have widely spaced molecules and atoms that are not as tightly packed as those in liquids or solids. This means that there is more space between the molecules for neutrons to pass through, making it less likely for them to be absorbed.
As mentioned, gases have widely spaced molecules and atoms, which means there is less surface area for interactions with neutrons. In comparison, liquids and solids have more closely packed molecules, providing a larger surface area for interactions and increasing the likelihood of neutron absorption.
Yes, some gases, such as helium and hydrogen, have lower atomic numbers and fewer electrons, making them less likely to interact with neutrons. In contrast, heavier gases like xenon and krypton have more electrons and are more likely to absorb neutrons.
Yes, temperature can affect the ability of gases to absorb neutrons. As temperature increases, the molecules of a gas move faster and become more spread out, making it less likely for neutrons to be captured. This is why gases are often used as coolants in nuclear reactors.
The pressure of a gas can also impact its ability to absorb neutrons. Generally, at higher pressures, the molecules of a gas are closer together, providing a larger surface area for interactions with neutrons. This means that higher pressure gases may be more effective at absorbing neutrons compared to lower pressure gases.