For those who don’t know, I have a bit of a side hobby fixing and using radiation detection equipment, such as Geiger counters. If you’re not into amateur science and nuclear science/energy, this may not seem very exciting.
Geiger counters and other basic radiation detectors are great for getting a general idea what the background radiation is or finding radioactive materials. However, they have their limitations. A Geiger-Muller detector only tells you when a gamma photon (or alpha or beta particle) is detected. It does not provide the energy level of the radiation. Since Geiger-Muller tubes respond differently to different energy levels of gamma emissions, it’s difficult to get a completely accurate assessment of what the dose rate is. A rough approximation is still possible, but a reading of the energy levels provides much more information.
In addition to better dose estimates, being able to measure the energy levels of gamma photons allows for identification of isotope which is being detected. Gamma-emitting isotopes produce emissions at characteristic energy levels, and by measuring these energy levels it is possible to determine what kind of isotope is present. It is also possible to tune a detector to measure only the desired energy levels and thereby pick up on a desired isotope’s emissions.
Bellow is a graph showing gamma measurements of a variety of radioactive materials.
To do this, two things are required. The first is a detector that is capable of measuring the energy levels of the radiation emissions. The second is an instrument that can record and display those energy levels, rather than just the raw count rate of the detector.
Detectors that can be used for spectroscopy include semiconductor detectors, gas proportional detectors and scintillation detectors. Of these, semiconductor detectors provide the best resolution and precision for measuring radiation levels. Unfortunately, for a semiconductor detector to work properly, it must be kept extremely cold. For this reason, a tank of liquid nitrogen is required, making it cumbersome and non-portable. There are also some new semiconductor detector setups that have an integrated refrigeration system, but these are very expensive. Because of this, semiconductor detectors are not appealing for field work or amateurs. However, they are the mainstay for laboratory setups.
Gas proportional detectors can also be used, although they produce less high resolution measurements. Still, a good gas proportional detector can provide measurements that are adequate for spectroscopy, especially when the radiation flux is relatively high. Gas proportional detectors do have their own limitation, however. Most gas proportional detectors require a constant supply of gas being feed to the detector. This is cumbersome for field work and requires the purchase of counting gas. A common mixture is 90% argon and 10% methane, so it’s not an exotic or difficult to obtain gas mixture, but it does need replenishment as the gas escapes the detector. There are sealed gas proportional detectors, but these are less common and mostly used for neutron detection.
Scintillation detectors work by converting gamma energy to light and then measuring the light. A material called the crystal is used. When a photon strikes the crystal, it produces a brief and faint flash of light. The crystal can be any number of different materials. Not all materials are actually crystal. Plastic and doped glass, for example, can be used. For high resolution measurements and spectroscopy, thallium doped sodium-iodine is common. Scintillation materials can also be liquid or gas, with liquid scintillation commonly used in laboratory analysis.
The light produced by the scintillation crystal is detected by a photomultiplier tube, which is a highly sensitive photon detector. A few newer scintillation-based detectors use photo diodes, but they don’t provide the sensitivity of a photomultiplier. When light is detected, the photomultiplier produces a pulse which is proportional to the energy level of the gamma photon. Scintillation detectors have excellent sensitivity and are versatile, being useful with a ratemeter for simple surveying or with a multichannel analyzer. They are inexpensive and rugged. Although they lack the precision of semiconductor devices, they are adequate for most spectroscopy work, such as identifying isotopes.
There are two basic types of analyzers: single channel and multichannel. Single channel analyzers work like a regular ratemeter, such as would be used with a Geiger-Muller tube. However they allow the energy level of pulses counted to be chosen. The user of an SCA could therefore dial in something like one hundred thousand KeV (kilo electron volts) to two hundred KeV. The counter would only respond to energy levels within that window. This can be useful for searching for a particular isotope, while ignoring background. It’s also possible to identify isotopes by measuring them with different detection thresholds, to establish what energy level is being emitted. Single channel analyzers have largely been replaced by multichannel analyzers.
Multichannel analyzers are much more capable. They have a large number of different channels and can measure energy levels across the gamma spectrum. It’s not atypical to have something like 2048 channels of analysis. The multichannel analyzer graphs the energy levels across these channels to produce a displayed spectrum of the measurements. This is useful for dosimetry, accurate measurement of activity and other purposes. One of the most useful features is the ability to identify isotopes. By comparing the detected emissions to a database of energy levels, it’s possible to determine what radioisotope or radioisotopes are present.
In the past, analyzers were large and complex pieces of equipment. They typically included a number of different components, such as signal amplifiers, analog to digital converters counters and pulse gates, often mounted into a NIM (nuclear instrument module) bin. Early on, multichannel analyzers were limited in the number of channels because each channel needed a separate counter.
Later, self-contained analyzers became available. By the 1980′s they had taken over for multichannel analysis. They may look like a small computer and often include a high voltage power supply, to power the detector. This greatly simplifies the equipment necessary. There are also portable MCA’s, which can be used for measurement and identification in the field.
By the 1990′s, PC-based MCA’s were common. These were simply a computer with spectroscopy software and a card or external device that captured the signal. These have advantages such as the ability to save spectrographs and download isotope databases.
Unfortunately, all these options are expensive. Stand alone analyzers typically sell for thousands of dollars. If you are looking for one that is used, you might snag one on eBay for a few hundred. Computer-based systems are not much cheaper. Because it is such a specialized area, the hardware and software can cost into the thousands.
But things have changed!
The basis of a digital multichannel analyzer is an analog to digital converter, which samples the pulses and provides digital measurements which can be analyzed with hardware or software. The equipment used to be quite expensive, but today, computers ship with a suitable ADC in the sound card. Sound card spectroscopy requires only a detector and high voltage power supply. An external amplifier may also be required, but they are not expensive.
There are even portable setups, with the detector connected to a smartphone. One can make a dedicated MCA for just $79 using a cheap portable oscilloscope device. Software exists which is free and open-source.
One simple and relatively inexpensive way of getting started is to use a device made by Gammaspectacular. It is a soundcard interface and detector power supply that is simple and only costs about $300. This plug-and-play system is easy to setup and much cheaper than systems of the past. Of course, it is also possible to build one’s own interface, which is relatively simple. Suitable HV power supplies are also not hard to find or expensive.
The one part of such a system that is not so easy and cheap to get is the detector itself. Basic scintillation detectors can be found on eBay for a couple hundred dollars. There are also a few scintillation detectors that are available surplus and are pretty cheap. A detector for low energy use can be found on this page for just two hundred dollars.
It’s also possible to build a scintillation detector and not very hard to do. It’s just a matter of pairing a suitable scintillation crystal to a photomultiplier and sealing it so no external light can enter the detector. Photomultiplier are not expensive, with many available for less than $50. Photomultiplier bases can be as simple as resistors soldered to the output pins. There are also ready made photomultipler socket bases. A seller on eBay offers them for $40. Suitable crystals are not hard to find either. Plastic-based detector mediums are cheap and sodium-iodine scintillation crystals are available for relatively low cost. In general, larger crystals tend to cost more. High quality Russian-manufactured crystals can be less than one hundred dollars, for a relatively small one. Again, eBay is your friend here, as it’s a great place to find odd surplus items for use in radiation detection.
It’s also possible to scavenge suitable scintillation components from old equipment. That is how I put together an NaI(Tl) scintillator. I took the crystal from a PRI-111B scintillator. These detectors were the high-end of radiation detectors sold during the 1950′s uranium boom. They have excellent scintillation crystals. I got the detector for about $50. It did not work and had heavy corrosion in the battery compartment. Even the dial was broken. But the crystal was still good. It had become opaque and yellow, due to exposure to moisture, which is common in old scintillation crystals. However, there is a website that offers to rework the crystals back to like-new condition for less than 30 dollars. I have no idea how it is done, but I believe it involves putting the crystal in a vacuum chamber and possibly heating it, to draw out moisture. Unfortunately, the website that offered this is gone, but if anyone needs the service, I believe I have the contact information for the gentleman who provides the service.
EDIT: I have found out that the individual who did the crystal reworking has died. However, it should not be too hard to figure out how the crystals are dried and renewed.
Links and Info:
Check out these links for information on home spectroscopy, system descriptions and instructions
Gamma Spectacular - maker of a nice little interface box for sound card spectroscopy
DIY Geiger Counter - includes information on scintillation detectors and spectroscopy
Gamma Explorer – A blog on spectroscopy
DIY Physics - Highly recommended site of advanced physics projects and numerous articles about spectroscopy with cheap and homebrew equipment.
Special Nuclear Materials - Carl Willis’s site on amateur nuclear-related projects including spectrometry
This entry was posted on Wednesday, September 17th, 2014 at 6:44 pm and is filed under Good Science, Misc, Nuclear. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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