FAQ - The care and feeding of PMT's and NaI crystals

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Richard Hull
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FAQ - The care and feeding of PMT's and NaI crystals

Post by Richard Hull » Wed Feb 18, 2004 4:04 pm

PMT is a contraction for PhotoMultiplier Tube. The PMT, by itself, is pretty worthless as it is usually just part of a process. Its duty is to convert light to electricity in a very special way. This effort in the PMT involves two processes in the one device and they are the photo-electric effect and electron multiplication.

How the PMT works:

When a photon of light, Ideally of a specific wavelength, strikes the face of the tube, it impacts a photo-cathode made from a vapor deposition of an alkali metal compound. This action releases an electron which is accelerated towards a positively charged first "dynode". This dynode is often coated, but may not be depending on the tube.
The now fast moving electron slams into the dynode and realeases a number of electrons in the process. These electrons then are accelerated to a second, more positive dynode and, impacting there, create a torrent of electrons. The process proceeds in repetition on through 8 or even up to 12 other dynodes! Needless to say, by the time the massive storm of multiplied electrons hits the anode, a rather large current pulse is seen. This is further amplified and a formal electronic pulse heralds the impact of the original single photon.

Big deal! We are not looking at blue photons, we are looking at Gamma rays! We need a "down converter-absorber". This is satisfied by a perfect crystal of Sodium Iodide that is Thallium activated, (much like a phosphor). Gamma rays entering this crystal will impart a characteristic energy to photons of the active sites which is of a specific wavelength. (blue)

The number of these blue photons is directly proportional to the energy of the original gamma photon, i.e., more energetic gammas will release more blue photons as they travel through the optically transparent crystal.

Adding a crystal to a PMT:

If we slap one of these special crystals onto the face of the PMT, we now have not only a gamma detector, but a gamma energy grader/detector. The more energetic the gamma, the more blue photons, the stronger the current pulse at the anode of the PMT. With appropriate circuitry, we can separate into "channels" these energy signatures and determine a lot of information about the source of the gamma radiation.

It is most important that the crystal be flawlessly "optically coupled" to the PMT. This is usually accomplished by a form of silicon compound which is like a jel and is smeared all over the faces of the crystal and PMT to act as a light transfer medium of similar refractive index to the glass and crystal.

Problems and proper care of a PMT:

The PMT, per se, is very robust and is no more delicate than any other vacuum tube. It does have one major caveat and that is that, under power, it cannot be left open to normal room light or it could be instantly damaged as the anode and dynodes try to carry nearly infinite photo current. In short, the PMT should forever be kept in total darkness. This is usually the case, anyway, as it is always coupled to an optical crystal or other medium where only a few photons might normally be encountered when in operation.

Crystal Issues:

The Sodium Iodide crystal is another matter all together! This is a very delicate item, indeed. It is hyper sensitive to even small physical shocks, low levels of moist air, and environmental temperature gradients.

As normally encountered, these special crystals are very tightly packaged in aluminum or statinless steel cans whose walls are painted white inorder to reflect every photon back out of the "open face" of the crystal. The ultra-tight fit also seals out moisture and other chemical contaminants from the environment. Even a tiny bit of moisture will cloud the otherwise optically transparent crystal over time and, in doing this, will ruin it for spectrometry use.

This crystal is no different from any other solid in that it expands and contracts with varying temperature. As it is tightly wedged in a metal can, too rapid an expansion of the crystal body can crack the crystal. This is mostly due to a change of environment where the crystal undergoes too rapid a warmup.

Likewise, too cold an environment will make it contract and the reflective coating on the can can peel and split off. Cracked crystals will not perform well and photons will be reflected and diffracted away from the window making a spectrum muddy and diffuse.

Physical shock will crack the crystal in a typical, glass like, concoidal fracture pattern. This is very bad and even a light tap will shatter the cyrstal, rendering it worthless.

SO .............

1. Never let a PMT/Crystal combo ever see a rapid temperature shift. Keep them at room temperature or very close to it.

2. Treat the detector head assembly as if it were a newborn baby. Handle it very gently not only in the infrequent transport or movement, but also in day-to-day use.

3. Never "break into" a crystal housing or defeat its moisture barrier as just the admission of a tiny bit of moisture will spell the end of the detector.

4. Never apply power to a photomultiplier tube that is exposed to even very low levels of light. IT MUST BE IN TOTAL DARKNESS!

Used PMTs:

This a real crap shoot. Combination crystal and PMT heads abound on the internet sites, e-bay and labX. Most sellers just will not know the condition of the device and, if it is surplus, it is most likely bad. Due to the above problems, one can't know the history of the detector head after it was surplused. Was it left out in a scrap yard to the elements? Left in an unheated warehouse? How many times was it banged or dropped in the junk yard or storage facitilty by hands that were used to handling tonnage of junk and not delicate laboratory instruments? You get the picture, I am sure. Bad surplus detectors are almost always related to a shattered, cracked or occluded, milky crystal.

I hope this FAQ will help others in dealing with the intricacies of obtaining and protecting a nice PMT/Crystal combo.

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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