BF3 tube energy spectrum

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Carl Willis
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BF3 tube energy spectrum

Post by Carl Willis » Thu Oct 06, 2005 4:15 am

The following data were collected with a Nancy Wood G-10-2 BF3 tube housed in a 3" Cd-lined, HDPE moderator (Ludlum 42-9), exposed to a very weak AmBe neutron source. This is a pretty nice modern BF3 tube, in contrast to the antique tube I showed on 2005-02-03, and so gives more of a "textbook perfect" response.

This graph illustrates nicely the physics of the B-10 neutron detection reaction.

The response has three distinct plateaus of sorts, which I've labelled A, B, and C.

Region A is counts that come from full absorption in the active volume of the 0.84 MeV Li-7 product and only partial absorption of the energy of the 1.47 MeV alpha particle (the rest of whose energy is expended in the tube walls). In Region B, which lies above the 1.47 MeV alpha energy, we have events possible in which the full energy of the alpha and less than full energy of the Li-7 nucleus are absorbed in the tube gas, in addition to the same type of events in Region A. Hence the plateau in this region has more counts. At the high-energy end of Region B is the "full energy peak" which results when the energy of both reaction products are fully absorbed in the tube gas (the detection event was probably close to the center of the tube). Finally, Region C represents the situation in which the B-10 reaction goes to the ground state and produces higher-energy charged particles (occurs in about 6% of events). In the other 94% of reactions, a 479 keV gamma is released whose energy escapes the tube.

Below 0.84 MeV, counts from neutrons are possible but very unlikely. This is because the kinematics of the B-10 reaction causes the products to fly off essentially in opposite directions and if one is lost to the tube wall, it is very likely that the other will deposit all its energy in the active gas volume. So to count neutrons and not pick up any gamma response, the LLD should be set near where the red dotted line is seen, somewhere below the 0.84 MeV Li-7 cutoff.

Also noteworthy is that the gamma / noise tail is very steep. This suggests that if you don't have an MCA and are setting up a BF3-based detector, one way to get the LLD set about right is to stick the bare tube next to a strong gamma source (like a piece of uraninite or a aircraft radium dial) and crank the LLD up until the rapid countrate just stops. This tube normally chirps about once every 10-15 minutes from neutron background, but with the neutron source nearby it goes off about once every 15 seconds.

The other equipment used was my Tracor-Northern MCA, Ortec 571 amp, 1000 kV bias voltage, and an anonymous charge-sensitive Ortec preamp of '70s vintage.

-Carl
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Frank Sanns
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Re: BF3 tube energy spectrum

Post by Frank Sanns » Sat Oct 08, 2005 3:30 pm

Very nice Carl! I vote for an archive.

Interesting how the energy barriers' transitions are quite sharp. What are the chances of you doing a similar spectrum with your large He3 tube? Do you think the resolution would be good enough for neutron spectroscopy?

Frank S.

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Re: BF3 tube energy spectrum

Post by Carl Willis » Sun Oct 09, 2005 7:22 pm

Hi Frank,

I've taken a spectrum with the He-3 and it is up here...somewhere. I'm certain it is in this forum and not Img. du Jour (by the way, everything not on Image du Jour is automatically "archived"). I also have a comparison of He-3 neutron detection spectra for different amplifier shaping times. Maybe I didn't put that graph up.

The He-3 tubes can do neutron spectroscopy over a limited energy range. However, the cross-section of the reaction falls off as the neutron energy goes up. I haven't really tried spectroscopy with my tube since I don't have any source of neutrons that would give a distinct spectrum. BF3 tubes could conceivably be used for neutron spectroscopy but only with some kind of complicated unfolding program.

-Carl
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Re: BF3 tube energy spectrum

Post by AnGuy » Tue Oct 11, 2005 1:06 am

>Also noteworthy is that the gamma / noise tail is very steep. This suggests that if you don't have an MCA and are setting up a BF3-based detector, one way to get the LLD set about right is to stick the bare tube next to a strong gamma source (like a piece of uraninite or a aircraft radium dial) and crank the LLD up until the rapid countrate just stops

Question, with my BF3 tube (1 inch by 12 inch) I get no noticable hits from a unranium ore. I get about 1 to 1.5 hits per minute from background with or without the ore sample. It was my understanding that BF3 did not respond to gamma radiation. Is this assumption incorrect? Now the HE3 tube on the other hand is very sensitive to gamma.

How are you translating hits into MeV with the BF3 tube? Pulse height? From my observations with background, all the pulse heights appear to be the same.

The only interesting thing I noticed, is that occationing the there will be a double pulse hit. I assume that this is the same neutron (or cosmic ray) striking twice. I though one might be able to roughly measure the neutron's speed by measuring the time between the two pulses, and use the tube length as the error margin. Of course this isn't very precise, since the two pulse might occur less than a millimeter apart and we don't know how much energy was lost during the first strike.

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Re: BF3 tube energy spectrum

Post by Carl Willis » Tue Oct 11, 2005 3:12 am

Hi AG,

First of all, nice tube. That's a big one. I suppose it's an N. Wood?

If your detector gain is high enough, you will indeed see the gamma counts if the tube works. Any tube will see gammas to some degree because of liberated electrons and ions formed by the rays' passage. The insensitivity of the BF3 that is commonly referenced has to do with the large Q-value of the boron reaction with neutrons, which makes it easy to separate these events from gamma events. We don't mean that the tube itself will not detect gamma events. You probably just have to increase the gain. Gain is a function of both the gain pot setting on the counting instrument AND the gas amplification factor of the tube, basically corresponding to the bias voltage. If you cannot get gamma counts to register on the highest gain setting / lowest LLD setting, turn the voltage up some. You might also want to use a shorter cable (loading the detector with a large cable capacitance degrades the signal) or a higher load resistance (100M for instance).

The channel-to-energy calibration is done by noting that the "stair-steps" of the differential energy spectrum correspond to the kinetic energy of the lithium nucleus and alpha particle, respectively, and that the peak is at the sum of E(Li) and E(alpha). Seeing this clean response requires that you have low noise, linear pulse-processing electronics behind the detector.

I haven't taken a long-term background spectrum with my BF3, so I don't know what a background response looks like. I'd guess that you'd still see the neutron-related spectrum from natural neutrons and that anything else is noise or comes from natural alpha contamination in the tube.

The double-pulse issue isn't a neutron "striking twice," it's probably an electronic problem. (Once a neutron is absorbed by boron, it is gone.)
Double-pulsing can come from insufficient quenching or too high a voltage, maybe also from a very-badly-set pole-zero control, or even a long coaxial cable between some elements of your setup that is not terminated in the right impedance.

I'm curious to know more about your setup. Maybe you've posted something on it before. It sounds like you have the right elements in place.

-Carl
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Re: BF3 tube energy spectrum

Post by Richard Hull » Tue Oct 11, 2005 2:41 pm

I am assuming annoymous guy has a digital scope and is looking at the pulses. If so, then the gammas are obvious with the approach of a mineral or radium source. I use only this method for setting my discriminator after the preamp stage. Without a scope, you are just going to have to tough it out by dialing in the lower level pot until all source gamma detection disappears. A digital scope usually lets you accurately set an intelligent, lower level, "noise margin window" quantitatively related to an observed maximum gamma pulse height on the scope.

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Re: BF3 tube energy spectrum

Post by AnGuy » Wed Oct 12, 2005 4:25 am

>First of all, nice tube. That's a big one. I suppose it's an N. Wood?

No, its a brand new Centonic BF3 tube (supplier in the UK)

>If your detector gain is high enough, you will indeed see the gamma counts if the tube works.

I suppose that the noise ( <4 mv) could include gamma hits. I assumed that anything under 24 mv is noise generated from the input power supply, the HV module, or general emf noise. The pulse hits that I assume are real are about 50 mv. I will have to take a closer look next time and see if I I can spot gamma pulses. BTW the 4 mvV and 50 mV is (raw output from the tube, before the JFET). I don't remember what the 50 mv pulses were after the JFET amp I think they were near 2 Volts. I haven't been working with the BF3 tube because I been playing with a HE3 I recently aquired.

>The double-pulse issue isn't a neutron "striking twice," it's probably an electronic problem. (Once a neutron is absorbed by boron, it is gone.)

Thats what I originally thought but these double pulses are distinct. For instance there are periods of minutes of flat line and then the double pulse occures with in less than a 1 usec with similar pulse heights. When I see noise (> 10 mV), I see dozens of pulses that are quite chaotic (random pulse heights and envelopes). This occures when a device (such as a refig) kicks on or off. I don't have a digital scope so I can't show the actual waveform but the way it appeared didn't see like electronic problem. I suppose that it could be a glitch in the oscilliscope, but that seems far fetched. these double pulse are also extremely rare. Perhaps once every few hours. I would guesstimate that one out 80 pulses is a double pulse. Could it be that when these double pulses occur, that the neutron is striking a non-boron atom (ie fluorine, containment, etc)?


>I'm curious to know more about your setup. Maybe you've posted something on it before. It sounds like you have the right elements in place.

Its pretty simple. HV+ 100Meg HV resistor (TRW) to MHV connector to Tube (HN connector) with a 2.5 RG6 cable (98+% shield copper cable). The MHV ground is connected to a 1Meg 1/4 resistor. I have a pair of 100 pf capacitors that dump into a JFET op-amp (with a 5 V zenor Diode for protection connecting the outputs of 100 pf caps). The pp noise is about 4 mV from the HV source. The pulse heights are about 50 mV +/- 10 mV (pre- JFET Op-Amp, direct from the output of the 100 pf caps.)

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Re: BF3 tube energy spectrum

Post by Alex Aitken » Fri Oct 14, 2005 3:11 pm

Just a quick check, but your preamp is charge integrating, right?

I'm wondering if you are running the tube voltage too close to the plataeu and getting pulses from failed quench events. Just a thought.

Pulse height (maximum) of a charge integrated output is the usual method. The decay time of the charge sensitive stage is usually set to be significantly larger than the time a pulse actually lasts coming from the device.

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Re: BF3 tube energy spectrum

Post by AnGuy » Thu Oct 20, 2005 2:19 am

>Just a quick check, but your preamp is charge integrating, right?

No, just a JFet Op-Amp configured as a inverting amp. I see the same waveform that the tube puts out, just amplified from millivolts to volts.

Question, what happens when Neutrons are slowed down? Boron is an effective moderator right?

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Re: BF3 tube energy spectrum

Post by Richard Hester » Thu Oct 20, 2005 6:50 am

The moderating is done before the neutrons reach the BF3 tube. That's why commercial neutron detectors have a big hunk of polyethylene around the BF3 or He3 tube. The boron inside the detector tube is not really dense enough to do much moderating.

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