button sized neutron detectors

[AllenWallace]

Argonne has developed a small neutron detector. see http://www.anl.gov/OPA/local/news/an020701.html#story1

I figure that this group could come up with a variation of the design. Can we smear lithium grease on a photocell?

[Richard Hester]

Boron or lithium based detectors, even when coupled to a semiconductor detctor such as GaAs, only respond efficiently to thermal neutrons. The little button would need a big, fat moderator to work with the fast neutrons generated by a fusor.

[Richard Hester]

Take a look at the Marion and Fowler book on fast neutron physics - one of the first signs of excessive exposure to fast neutrons is cataracts. These happened distressingly often with physicists operating cyclotrons without taking proper precautions. Recoil protons wreak havoc on living tissue, and the capture gamma emitted when a neutron is finally captured by a hydrogen nucleus just adds insult to injury. I would prefer that all the havoc be wrought in a bucket of paraffin or HDPE, with the operator being at a distance sufficient such that inverse square lessens the radiation impact. The neutron dose from a 10^5 neutron/sec fusor is peanuts, but it pays to get get safe radiation habits ingrained in your system.

[Richard Hester]

I realize the value of a compact detector for contraband detection, but any neutrons from smuggled bomb-grade material will likely be fast ones due to spontaneous fission. The gamma signature, however, is an entirely different matter.

[Richard Hull]

Richard is correct If you hauled around 10 pounds of moderately enriched U-235 metal, you could get a beta burn or gamma blasted but never get a neutron hit. The stuff's spontaneous fission rate is about an order of magnitude less than plain old metallic U-238! So.........no neutrons and what few might occur in a large mass would most likely self absorb.

The deal with neutron detection comes with a suitcase sized nuclear device. Here the fissile material is still of little importance but the neutron source needed to start the stuff to cookin' is most likely within the device and producing slow neutrons. If not in the device, someone has to carry it in separately and they will still be detected by neutrons. As most sources are Am241/Be, a normal scintillator or geiger counter would not detect it...... (sources are usually in a thin lead or brass casing absorbing all of the Am241's weak 59kev gammas and ultra low penetrating 5.5mev alphas), but a neutron counter would pick this up easily. All this assumes a "dim bulb" tries to bring it in past heavy border or airport security. Anyone thinking we are dealing with "dim bulbs" needs to re-think their racial bias.

There is just about zero chance of a device being assembled here from internally available materials. The stuff would have to be smuggled in. I hope our tax dollars are paying for the equipment needed to detect whackos bringing stuff in from the outside.

We really need to do much more in sealing up the border traffic. Do you really think there is a 4 kilomile fence between the US and Canada? I worry that if we are hit, this is the way it will get in. Probably as a simple walk in at night. Too scary to even think about.

Richard Hull