Why do gases absorb neutrons less readily than water?

1. Mar 8, 2006

leila

Why do gases absorb neutrons less readily than water?

Any ideas on the above question?

Leila

2. Mar 8, 2006

silkworm

I would guess it's because of the same reason that a table is better than catching a drop of water than a net.

3. Mar 8, 2006

CarlB

I would think the issue has something to do with getting the momentums and energies to match. In a gas, I'm guessing that one can model the neutron / atom interaction a single atom at a time. The result is a difficult reaction because you have to match up the momentum and energies beforehand with the momentum and energies afterwards. That is very restrictive.

For a liquid, the target atom is not alone so the momentum can be got rid of by exciting other atoms.

Maybe thinking about the same situation in playing pool on an infinite pool table would help. With a gas of object balls, it is very easy to end up just touching one. But if there are so many object balls that they are bouncing against one another, then it is possible to quickly get rid of the excess momentum that happens when your cue ball comes in.

Carl

4. Mar 8, 2006

Staff: Mentor

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.

Xe135 has a very high thermal micrscopic cross-section for n-capture 2.647E+6 barns! http://wwwndc.tokai.jaeri.go.jp/cgi-bin/Tab80WWW.cgi?/data2/JENDL/JENDL-3.3prc/intern/Xe135.intern
http://wwwndc.tokai.jaeri.go.jp/cgi-bin/nuclinfo2004?54,135

The other part of this phenomenon is the macroscopic cross-section vs microscopic cross-section.

Each atom has an energy-dependent microscopic cross-section ($\sigma$) 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 ($\Sigma$). 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.

Last edited by a moderator: Apr 22, 2017 at 8:44 AM
5. Mar 9, 2006

mathman

In the orignal question, is density being taken into account? Gases can be made of many different substances, for example steam, which has the same neutron interaction properties as water, but being much lower density, requires more volume.

6. Mar 9, 2006

CarlB

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

Last edited by a moderator: Apr 22, 2017 at 8:46 AM
7. Mar 10, 2006

Haelfix

Lets just say its pressure, temperature, cross section and molecular interaction dependant (in stat mech they call this a potential) and thus complicated =)

But I tend to agree (grossly) with Astronuc in general.

8. Mar 10, 2006

Grogs

Carlb: Since that paper is dealing with 'cold' and 'ultra-cold' neutrons, I agree with your assessment of why the cross-sections of liquids and gases are different at those energies. The neutron wavelength is large enough at those energies to interact with multiple atoms rather than just a signle one. Solid carbon-12, for example, has a lot of resonances in its scattering cross-section around 5-10 meV where the neutron wavelength matches up with the spacing of the atoms in the crystal lattice. I could see how it's possible that, even after you correct for density, material of different phases could have different cross-sections. At higher energies, I would expect the cross-sections to converge.

I'm not 100% certain if that's what the OP was after though. If the question is why gases, in general, have a lower cross-section that liquids, then I like what astronuc said: they don't necessarily, once you correct for density.

Last edited by a moderator: Apr 22, 2017 at 8:47 AM
9. Mar 10, 2006

Staff: Mentor

It is that straightforward, although the calculations are not necessarily trivial.

Each nuclide (isotope) has a unique microscopic cross-section as a function of energy. Given the same isotopic composition and therefore same effective microscopic cross-section, gases have much lower atomic (molecular) density than liquid, say at room temperature to about 300-347°C and under pressure, and therefore lower macroscopic cross-section than liquids.

One has to 'know' the neutron energy spectrum (which is determined by the degree of 'moderation', which is a function of the core materials and their composition) and collapse the microscopic cross-sections, and use the 'appropriate' atomic densities to calculate the appropriate macroscopic cross-sections.

One does have to consider molecular structure at or near 'thermal energies', because molecular dynamics does play a role in the relative speed of neutrons and absorbing materials.

I am thinking the OP is addressing power reactor conditions, rather than 'cold' and 'ultra-cold' neutrons.

Last edited: Mar 10, 2006