Uranium-238 Fission Ion Chamber for Neutron Dosimetry (Confused? You s (in Off-topic)

SNK3R August 20 2007 9:37 PM EDT

So, I've just about finished up another internship conducting nuclear physics research. You can take a look, if you dare, at my research paper here. [Size: ~500 KB]

The layman's version? Read on...

Neutrons have a neutral charge. Thus, there is no "easy" way to detect neutrons. Therefore, indirect methods are used to accomplish this task. The neutron detector I tested, a fission ion chamber, indirectly counts the number of relative incident neutrons.

The way this happens is: neutrons collide into a fissile foil (uranium-238 in this case). When one event finally occurs (it doesn't always happen every time), the neutron collides into the uranium nucleus, producing an unstable uranium isotope. This isotope emits fission fragments. The chamber for which the fissile foil is contained also contains a gas (usually argon or methane). When the fission fragments (being very heavy) collide into the gas molecules, an ion path is created. These ions can then be collected on an electrode (also in the chamber), and the signal can be sent through electronics which detects a certain number of counts. The counts are represented by current (the amount of charge over a certain amount of time). These counts can then be related back proportionately to the incident number of neutrons. Thus, you can know how many neutrons were hitting the fissile foil.

Okay, so you managed to follow some or most of the above. Now, why would a neutron detector be important? Well, for several reasons. Using a particle accelerator (such as a cyclotron), we can simulate various scenarios that may occur in space, for example. So, to expand this example, neutrons bombarding with objects on space vehicles may cause a shut-down or malfunction of certain computer chips or parts. This can be a serious problem, of course. (By the way, this doesn't always happen in space, it's also known to happen via cosmic rays down to where we live, but the chance of it happening is greatly reduced because of the spacious atmosphere protecting us.) So, by using a cyclotron to generate neutrons and bombarding computer chips or parts with the neutrons at a facility, we can simulate if something is likely to malfunction or not BEFORE testing it in space. This, obviously, is a much more inexpensive way to test things, rather than spending billions of dollars on parts just to have it malfunction somewhere in space. There are additional uses of neutron detectors, but, personally, I feel this one is more important, so I won't go into the others.

I also did some efficiency tests using activation foils, but I won't explain it here, so see the paper for details on that.

Adminedyit [Superheros] August 20 2007 9:42 PM EDT

I'm still waiting for you to tell me you've mastered cold fusion in your kitchen sink.

Thraklight Resonance August 20 2007 10:05 PM EDT

I'm fairly certain that he has washed off his Gillette Fusion in the sink a few times.

[P]Mitt August 20 2007 10:08 PM EDT

Is there a standard to determine exactly how many neutrons are in the chamber?

As in, a standard that encompasses the size of the chamber, the density of the U-238, the number of counts given, and other energy-giving sources (like heat, electricity, etc.)

(I just read your layman's version, not your big huge paper, so if the paper contains this data, don't blame me for not reading :P)

Admindudemus [jabberwocky] August 20 2007 10:13 PM EDT

when you have an accident and become a super-hero, don't forget your friends here on cb.

neat stuff btw, thanks for sharing!

SNK3R August 20 2007 10:20 PM EDT


Not necessarily. Certain producers of fission ion chambers, for example, sell various detectors with different specifications. For example, if you go to LND Inc.'s site, you can see that there are two sizes of fission ion chambers available. And, really, the fission ion chamber that we purchased from them was custom-made with U-238, for the detection of high-energy neutrons, and not thermal (slow). (They also make them with other isotopes such as thorium-232 or uranium-235). U-235 is used for thermal neutron detection.

As for other specifications, usually the voltage is pretty standard. You need enough voltage to create ion paths and not too much voltage to create a double electron production effect. This could mess with your results a little. The thickness of the fissile foil and most of everything else is really determined by how efficient of a result you want. For example, the fission ion chamber we tested had an efficiency of 1 data count per 1,000 neutrons.

In order to test this efficiency, we used activation foils to compare results to make sure what the _real_ efficiency is. (A real-life example that relates this idea would be that you purchase a car that claims it has a gas-mileage of 40 MPG, but when you drive it, the best you can get is 30 MPG.)

So, basically, what I'm trying to say, is there's a standard _procedure_, but not really a strict standard on specifics of the fission ion chamber itself.

Hope this answers your question, Mitt. If not, feel free to keep asking. Thanks for reading.


Thanks for reading!

Xenko August 20 2007 10:43 PM EDT

I take it there is no easy way to just shield to computer components to avoid this problem altogether? Or alternatively, there is no way to shield the components without vastly increasing the mass of material that needs to be taken into orbit?

Interesting stuff at any rate. Nice work :D I'll stick with my molecules, they are much easier to analyse (unless you get a mixture... in which case they can be a pain).

AdminQBVerifex [Serenity In Chaos] August 20 2007 10:57 PM EDT

I just wanna say that it's pretty awesome that you wrote all that. I can now seriously say, "so I was talking to this nuclear physicist the other day.." And that makes my day! :)

That paper is pretty cool, I got about 3 pages in before I got distracted, but I'll try to read the rest.. If I can...

Mem August 20 2007 11:43 PM EDT

"This can be a serious problem, of course. (By the way, this doesn't always happen in space, it's also known to happen via cosmic rays down to where we live, but the chance of it happening is greatly reduced because of the spacious atmosphere protecting us.)"

This could be making this problem even worse. Not to mention the fact that we don't even know what such an event would do to all our fancy new electronic devices...

Now you need to tackle this issue, SNK! You'd certainly go down in the annals of scientific history then. Nice stuff, nonetheless.

SNK3R August 21 2007 12:00 PM EDT


That's exactly correct! In order to seriously block and shield neutrons, we really need lots of lead, which is quite freakin' heavy. And if you think of having a huge, gigantic lead "shield" around all of your computer components, it just doesn't seem feasible, especially for space travel. There are other types of protection such as poly bricks, which are much ligher (you can pick one up with one hand easily, unlike lead), but they don't do as good of a job from neutron protection. It's a give/take scenario -- which should scientists focus more on? Tough question.


I'm by no means a "nuclear physicist", though I am doing projects in that area. Whatever makes your day is fine with me. ;) Thanks for reading. If you have any questions, feel free to ask.

QBsutekh137 August 21 2007 12:10 PM EDT

Great stuff! I only managed atomic research in my day... No nucleus crashing involved. But it is neat to hear about the similar methods (shooting particles at various things and then taking electronic readings... The guy I worked with had a "Wood's tube" full of hydrogen plasma (run huge voltage across it to make a cool pink fluorescent light bulb!). Then he shot ions at it (remember, atomic physics, not nuclear) to see how much ionized H2 there was...

Excellent paper, thanks for sharing!

SNK3R August 21 2007 10:20 PM EDT


Thanks for reading. :)

Thanks to NSFY, a PDF file is now available in lieu of the .doc. The size is only 500 KB now, so dial-up users have a chance to check it out of interested. Thanks NSFY! You can find the PDF file at the very top of the thread.

[P]Mitt August 22 2007 12:49 AM EDT

Since neutrons have no charge, it isn't possible to deflect or repel them from a possible target with a magnet. But is it possible to temporarily charge the neutrons with a positive or negative force that a strong enough electromagnet or something of the source will be able to change its trajectory?
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