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KarenRei

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I've been going over the cross sections on Sigma, and I'm a little confused as to why beryllium is the most talked about neutron multiplier I've come across. I mean, it does have a few things going for it: multiplication down to lower energy levels than most multipliers, and a very low (n, gamma) cross section at thermal energies (given that one expects a large portion of neutrons to thermalize in the multiplier, that matters). But its multiplication levels at 10MeV and above are unimpressive versus many heavier isotopes, and its light atomic mass looks like it should ruin its practical multiplication efficiency since neutrons will lose so much energy per elastic scatter and they'll elastic scatter more often than they'll multiply. And while beryllium is light, strong, and rather heat tolerant, it's expensive and toxic. And its low density works against it by decreasing its macroscopic cross sections.

Why not heavier elements? Zirconium, for example, also has a low (n, gamma) in the thermal spectrum, a higher cross section at energies over 12.5MeV, and neutrons lose 1/10th as much energy per elastic scatter, and it costs about 1/8th as much as beryllium and is relatively nontoxic. At about 1/3 the cost of beryllium ($300/kg according to one paper I read) one can get nearly isotopically pure 90Zr, which has an even lower (n, gamma) cross section. Natural lead has a rather high (n, gamma) cross section, but 208Pb has a very low one. Lead from thorium ores can be found naturally enriched up to about 90% in 208Pb, and it can be manually enriched beyond that. Lead not only has a very heavy nucleus, meaning little elastic scattering loss, but also gives a wide range of neutron multiplication reactions, even with a high cross section for (n,4n) at some energy levels. These are of course just two examples. Heavier elements also have other advantages, like usually being better gamma absorbers.

Am I missing something? Do heavy isotopes have abnormally high inelastic scattering losses that beryllium doesn't or something? The cross sections aren't bigger, at the very least. Or are people normally not that concerned with multiplication of neutrons over 10MeV? I guess if you're dealing with fission where the average neutron energy is under 1MeV perhaps you don't care about the higher energy stuff and only care about having a low energy multiplication cross section, versus fusion (17 MeV) or spallation (30-40MeV) where your source is much higher energy neutrons.

Could anyone clarify the situation for me?

Why not heavier elements? Zirconium, for example, also has a low (n, gamma) in the thermal spectrum, a higher cross section at energies over 12.5MeV, and neutrons lose 1/10th as much energy per elastic scatter, and it costs about 1/8th as much as beryllium and is relatively nontoxic. At about 1/3 the cost of beryllium ($300/kg according to one paper I read) one can get nearly isotopically pure 90Zr, which has an even lower (n, gamma) cross section. Natural lead has a rather high (n, gamma) cross section, but 208Pb has a very low one. Lead from thorium ores can be found naturally enriched up to about 90% in 208Pb, and it can be manually enriched beyond that. Lead not only has a very heavy nucleus, meaning little elastic scattering loss, but also gives a wide range of neutron multiplication reactions, even with a high cross section for (n,4n) at some energy levels. These are of course just two examples. Heavier elements also have other advantages, like usually being better gamma absorbers.

Am I missing something? Do heavy isotopes have abnormally high inelastic scattering losses that beryllium doesn't or something? The cross sections aren't bigger, at the very least. Or are people normally not that concerned with multiplication of neutrons over 10MeV? I guess if you're dealing with fission where the average neutron energy is under 1MeV perhaps you don't care about the higher energy stuff and only care about having a low energy multiplication cross section, versus fusion (17 MeV) or spallation (30-40MeV) where your source is much higher energy neutrons.

Could anyone clarify the situation for me?

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