FAQ - Neutron detector setup and null testing

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Richard Hull
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FAQ - Neutron detector setup and null testing

Post by Richard Hull » Fri Oct 18, 2013 5:23 pm

This FAQ will assit in helping you set up a neutron detecting tube assuming you have NO NEUTRON SOURCE. Only three tubes are considered.

1. Boron lined tube - (GE is a major manufacturer of these during the cold war. One of the cheapest, still to be found, neutron detector tubes)
2. BF3 tube - (The first high tech, useful neutron detector for very low flux measurement - Saw its first service in WWII manhattan project work. Old warhorse, but back in the limelight due to severe and crippling 3He shortage)
3. 3He tube - ( The finest of all neutron detection tubes, but now rare and exceedingly expensive, if you can find one)

It is assumed you have one of the tubes above and all the proper electronics such as a variable high voltage bias power supply, preamp, amp with an adjustable lower level discrimination circuit involved and digital counter. Note: All these tubes use a POSITIVE bias supply.

You can often just set your tube to not detect strong gamma rays and not need a true neutron source. Note: This method is a bit of a bypass method and may or may not be successful in setting you up to be sure of counting neutrons with the above tubes, but without a neutron source this is all you have to work with.

Things you need to know and consider.

1. What is the counter tube's shell made of? (The gammas you use in testing must absoltuely make it through the tube's shell!)
2. Will my gamma ray source penetrate the tube's shell? ( If #3 here is addressed properly you will be OK)
3. What is my gamma source and what are its gamma energies. (The source's gamma energies must be more than capable of penetrating the shell and depositing enough energy to fool the detector into counting gamma rays.)

GAMMA SOURCE:

If no neutron source is available you MUST acquire a gamma source that is fairly strong and whose gamma energies are high. X-ray sources are no good here unless they are at least in the 100kev range.

1. The ideal, safe source is a sealed cesium 137 source of about 10uCi producing 661kev gammas (maximum activitiy you can acquire in a sealed source in the U.S.)
2. An even better source is a very strong radium sample. These produce gamma energies as high as 2.6 Mev! However most of the energies are in the 200kev to 600 kev range. The only way you can legally obtain such a significantly intense source is in the form of an old, surplus, WWII military aircraft instrument. Note! Leave this meter sealed to stay legal! The meter glass face will not stop any of the significant gamma rays you need. A small simple pair of watch hands might not be potent enough to do the job here. You need a good heavily loaded military instrument dial face and hands which are often 50 times more potent
3. A third source that is very legal is a very intensely radioactive uraninite ore sample. (35,000 cpm+ on a CV-700 GM counter) You are using the radium and decaying daughters contained within it. It is wise to seal it up in two separate polyethylene bags to capture all of the radon daughters and their gamma ray decaying daughters. (intensifies this source of gamma rays.)

Americium sources are of little value here. Their 59kev gammas are just too weak.

Now you have a good gamma source........ Let's begin

INITIAL COUNTING SYSTEM AND DETECTOR SETUP

Power off, no detector tube hooked up

1. Turn your bias supply down to 0 volts
2. Turn you level detector all the way down to zero (000) and then back up to only about 10 or 20% of maximum. 100 or 200 on the ten turn pot of a nim bin.
3. Connect your detector tube to your preamp or counting circuitry.
4. Turn on your power supply, amplifiers and counter.
5. zero your counter

Tube specific instruction from here on............

BORON LINED TUBE

These tubes are tricky of bias adjustment. Too little bias and they will not be reliable counters, too much bias and they will act like a good gamma ray geiger counter. So be prepared to fiddle with this a bit.

1. Place your gamma source near the middle of the tube
2. Bring up the bias slowly until you start to get a rapid regular count. ( 5 counts per second or ~300 cpm). (this is usually between 500 and 700 volts but could be a bit more than 700 volts or but rarely less than 500 volts) Note and record this voltage.
3. Back off the bias voltage until your counter is still counting, but a good bit slower. (1 count per second or 60cpm) All this can be hyper tricky and sensitive of adjustment. Note and record this voltage.
4. Now raise the discriminator level detect level until you just stop getting frequent counts.
5. Remove the gamma source to a distance of over 10-20 feet
6. Reset the counter to zero and run a 10 minute background check. This should average out to about 5-10cpm at most. (cosmic rays/local gammas)
7. Replace the gamma source next to the tube.
8. Re-zero your counter and count for 10 minutes. The number between source and no source background should be close though a little increase might be OK.
9. Remove gamma source or readjust from step #4 until the difference is insignificant and you can't effectively detect the gamma source.

Here is where a decent neutron source would be very desireable. Note: you can't make a good neutron source out of the little 1 uCi smoke alarm americium sources in contact with beryllium! Even 10 of these little sources would not give a good neutron source, so forget it! If you attempt to use a super weak americium source, you might think you are counting neutrons when you are actually partially counting the 59kev gamma rays from the americium! These little 1 uCi sourced/beryllium source are worthless as neutron sources and can readily dupe the unprepared into thinking they are counting neutrons. FACT: a 50uCi Am241 neutron source might give a neutron count of 1 or 2 additional counts over background in a full minute of counting. A proper millicurie source of Am241 however might produce up to 60 counts per minute. This will vary due to the detector tube type and size, moderator assembly,etc!

If you have done the above and have no neutron source, you just might be OK with your adjustment and have to rely from this point on with neutrons from the fusor to tell you that your tube is working correctly.

NEUTRON SOURCE ....If you have a neutron source, place a 2 inch thick moderator block, (parafin wax, water tank, polyethylene next to the tube and then put the source on the other side and maybe sandwich it with another 2" block of moderator behind the source. Depending on the strength of the neutron source you should be seeing increase counts above background over a ten minute period. If so, you are good to go!

BF3 COUNTER TUBE SETTING

BF3 tubes are much better than a boron lined tube, however they typically take a lot more high voltage bias before they start working. (usually between 1200 and 2400 volts!) Some tubes have the maximum bias noted on the tube's serial number plate. You should try and obtain your tube's spec sheet on the internet using its part number or model number in a search. The BF3 tube and 3He tube demand a special charge senstivie amplifier which can be easily damaged if the HV bias appears too rapidly into the preamp's bias input jack. Thus, always bring up their "set bias" voltage slowly and, likewise, turn them down slowly before shutting down the system.

Use the same setup for the boron lined tube to adjust the BF3 tube, but do not let it go into discharge (hyper fast counting) when raising the bias voltage. If you get a discharge, immediately back off the HV bias to where you get only an occasional count or slow counting and then go to step #4 above and continue.

Read the neutron source info above if you have a neutron source.

3He TUBE SETTING

You are blessed and very lucky if you have this best of all neutron detectors. Most of these tubes have 4 atmospheres of pressurized 3He (helium 3) in them and most work best with their HV bias set between 1200 and 1800 volts though 2000 volts is not uncommon for higher pressure tubes. Again, look on the tube's serial number/model number plate for info or better still, do an internet search for you tube model's data and spec sheet. Both the BF3 tube and 3He tube demand a special charge senstivie amplifier which can be easily damaged if the HV bias appears too rapidly into the preamp's bias input jack. Thus, always bring up their "set bias" voltage slowly and, likewise, turn them down slowly before shutting down the system.

Set this tube up using the same procedure as the BF3 tube which is similar to the Boron lined tube. Again, never allow the bias setting to create a continuous runaway counting or discharge. BACK OFF THE BIAS...as noted in the cse of the BF3 tube.

Read the neutron source info above if you have a neutron source.

Carl willis posted some excellent videos on you tube showing how to simply and rapidly set up a 3He tube if you have a neutron source and NIM set up.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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