FAQ - Choosing a Gamma Spec Scintillator detector

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Richard Hull
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FAQ - Choosing a Gamma Spec Scintillator detector

Post by Richard Hull » Tue Oct 26, 2010 2:33 pm

A few of our group have Gamma Spectrometers, (gamma spec., for short), and more are coming on board each day. This instrument allows you to verify your neutron activation experimental results. This is all pretty advanced, but more and more here are entering the fray.

The scintillation head for most gamma specs utilize a photomultiplier with an optically mated crystal. This pair is sealed in an absolutely light-tight, EMI proof container or shell. The scintillion head for the gamma spec. is a critical item. 98% of us are using the standard Sodium Iodide, Thallium activated crystals attached to our photomultipliers tubes. (written NaI:Tl)

This head is usually connected by only three key cables to the electronics that generate the actual spectrum sent to it by the detection head. These are:

1. The high voltage. Usually about 1000 volts.
2. Ground. This is usually the HV low or return side and signal ground.
3. The shielded signal lead. This two conductor cable contains signal ground and the actual signal line inside the shielded, grounded sheath

There are three ways to get your hands on the critical detection head.

1. Buy one used. This is risky as many of these appear on e-bay and are not usable as the critical crystal is either cracked or cloudy making them completely unusable for gamma spec work. There is no way on earth to tell in advance if the offered head is good. Never buy a head used unless the seller gives you a money back guarantee that his offering is good. Such warranted deals might run in the range of $300.00 to $800.00

2. Buy a head brand new from the manufacturer. This is the ideal way to go but be prepared for a low price of $1000.00 to a high price of $2000.00.

3. Roll your own. You can readily assemble a complete head for under $200.00 with E-bay peruchases. You will need a photomultiplier, (PMT), and a crystal.

PMT's with socket and dynode string can be had for $50.00-$100.00 on ebay. The crystal is a bit harder, but NaI:Tl crystals in sealed metal containers show up on occasion for $25.00 to $100.00 The crystal and PMT should be of the same diameter though this is not an absolute requirement. The crystal, in the e-bay photo, should be water clear. If it is not do not buy it.

The crystal is joined to the PMT with a special optical grease or compound that allows the free passage of light. It will now be up to you to select a container that will shield the assembly from both light and electromagntic radiation and at the same time allow the wires from the PMT to exit this light-tight shell.

Note: It is common practice to place a Mu metal shield around the front region of the PMT. This is a special magnetic metal alloy in sheet form, wrapped as a cylinder extending from the tube face up to the anode region of the tube. This shunts any stray magnetic fields that might interfere with the electron multiplier in the tube.

Size matters.... to a point.................

In general, the larger the PMT and crystal or the pre-manufactured head, the better.

2" is a standard minimum with 3" being somewhat ideal. 5" heads give very quick resultant spectrums from strong sources and can count very weak sources readily, but these big heads are plagued by lower energy gamma noise which piles up fast. Small 1" heads miss a lot of the higher end gammas or take forever to collect a decent high end spectrum.

Crystal designations are given as "2-inch by 1-inch" or "2X1". The first dimension is the diameter and the second is the thickness. The best gamma spec crystals are "square". This doesn't mean square in shape, for most all crystals are cylinders. What the term means is 2X2 or 3X3, 1.5X1.5, etc. Most all manufactured, general use gamma spec heads are square crystal heads.

The diameter relates to the area of intercept. This means that the larger this frontal area is made, the more gamma photons will be entering the crystal for detection. The thickness or depth allows for a greater chance of being counted and is related to the energy of the gamma ray. The density of the scintillator always makes a difference in collecting photonic scintillations. This is why the ultra dense BGO, (see below), is so effective in detecting higher energy gammas. For any given scintillator crystal type and diameter, thicker is always better.

A 2X0.5 crystal would be great for counting weak, low energy gammas and allow the more energetic gammas to pass through virtually undetected. A 3X3 crystal would count many higher energy gammas that a thinner crystal would not even register.

What are the best of all worlds here?

A pre-manufactured 3X3 detector with mated pre-amp is the cat's PJs, Though a good 2X2 system is very useful and, in good hands, is all that most here will ever need.

If you are chasing just one gamma energy signature or one gamma energy range, this is where the wierd or oddball crystal dimensions will serve best. You are "tuning" the detector crystal to optimize catching your prey.

There are other gamma spec crystals and detectors. Bismuth Germanate, (BGO) is a great scintillator crystal for really high energy gamma rays, but has broad peaks in its spectrum and finds little use in general gamma spec work. BGO is also terribly expensive in any given size. Similarly expensive and just not generally found in amateur gamma spectroscopy is the use of Barium Fluoride scintillators.

Finally, the GeLi ("jelly") detectors are the best of the best for gamma spec work, giving needle sharp peaks and superb high energy captures. Unfortunately, these are ultra pro rigs and new costs exceed $5,000.00 and all require liquid nitrogen cooling. Rare e-bay finds can net one of these for a few hundred dollars, but the $3.10/liter liquid nitrogen bill usually ends the dream which evaporates faster than the LN2.

It is not to be part of this posting to cover the actual electronics box that generates the spectrum from these detection heads.

Richard Hull
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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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Steven Sesselmann
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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by Steven Sesselmann » Wed Oct 27, 2010 2:35 am

Richard,

You might want to add BaF2 or Barium Fluoride crystals, these come up on ebay occasionally and can be used to make a good scintillator probe. Unlike NaI crystals, they are not sensitive to humidity (unless heated).

Couple the crystal to a PMT with coupling fluid in the normal way, as you described above.

The dense crystals look like glass and are very heavy, they scintillate in the UV region of the spectrum, and have a very fast response time. With proper discrimination, these can also be used to detect mono-energetic neutrons.

http://en.wikipedia.org/wiki/Barium_fluoride

Steven
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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by richnormand » Wed Oct 27, 2010 10:38 pm

Nice work.

People getting into the game might benefit from reading:

R.L. Heath “Scintillation Spectroscopy Gamma Ray Spectrum Catalog” at

http://www.inl.gov/gammaray/catalogs/pdf/naicat.pdf

The intro is quite useful and explains a lot of the physics and geometry efects.

I also found “Radiation Decay" by Charles Hacker, Griffith University useful to hunt peaks (freeware program).

Keep up the excellent work. Much appreciated.

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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by Jim Stead » Wed Mar 23, 2016 2:14 am

richnormand wrote:Nice work.

People getting into the game might benefit from reading:

R.L. Heath “Scintillation Spectroscopy Gamma Ray Spectrum Catalog” at

http://www.inl.gov/gammaray/catalogs/pdf/naicat.pdf

The intro is quite useful and explains a lot of the physics and geometry efects.

I also found “Radiation Decay" by Charles Hacker, Griffith University useful to hunt peaks (freeware program).

Keep up the excellent work. Much appreciated.
I found the referenced Internet link to be dead. I believe the following is the same text http://sites.fas.harvard.edu/~phys191r/ ... atalog.pdf

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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by richnormand » Wed Mar 23, 2016 2:47 am

Jim Stead wrote:
richnormand wrote:Nice work.

People getting into the game might benefit from reading:

R.L. Heath “Scintillation Spectroscopy Gamma Ray Spectrum Catalog” at

http://www.inl.gov/gammaray/catalogs/pdf/naicat.pdf

The intro is quite useful and explains a lot of the physics and geometry efects.

I also found “Radiation Decay" by Charles Hacker, Griffith University useful to hunt peaks (freeware program).

Keep up the excellent work. Much appreciated.
I found the referenced Internet link to be dead. I believe the following is the same text http://sites.fas.harvard.edu/~phys191r/ ... atalog.pdf


Yes it is indeed the same. Very useful reference.
Cheers.

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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by Roberto Ferrari » Wed Mar 23, 2016 12:56 pm

Hi Richard

Your subject review is excellent, as always!
I have a question: what about the plastic scintillators?

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Richard Hull
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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by Richard Hull » Thu Mar 24, 2016 3:44 pm

Plastic scintillaors can never be used for Gamma Spec work. Absolutely unacceptable.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by Silviu Tamasdan » Thu Sep 28, 2017 11:49 am

I've seen lately creep up on ebay and elsewhere LYSO crystals (lutetium-yttrium oxyorthosilicate) which I believe are probably surplus from medical PET scanners. They are generally shaped into thin and long cuboids, and are intended to be assembled into multi-crystal arrays with each crystal constituting a "pixel" in the scanner's gamma camera coincidence detector. I intend to make and test one such array soon.
http://www.crystals.saint-gobain.com/si ... -sheet.pdf
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Re: FAQ - Choosing a Gamma Spec Scintillator detector

Post by John Futter » Thu Sep 28, 2017 6:09 pm

LYSO is very good
in fact it is nearly as good as NaI:Te approx 85% and is a ceramic so it does not absorb water
It is more dense so you do not need as much as the stopping power is much greater.
The light output is a beautiful Cyan colour and the lutetium dopant is radioactive so it has a builtin check source of a few counts per sec
i'll post a pic of it under x-ray bombardment

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