Hr. Hazwoper Software' title='40 Hr.Hazwoper Software' />How Do Sodium Iodide Scintillation Detectors Work Dear Dr.Zoomie your last posting discussed how gas filled detectors work, but Ive got a different type.I think its called a sodium iodide detector.Can you tell me how this works and when I should use it Thanks Youve got one of my favorite detectors and yes, I know that this raises me way up on Geek sale There are two fundamental families of detectors the gas filled detectors I wrote about the last time, and the scintillation detectors, of which the sodium iodide abbreviated Na.Issuu is a digital publishing platform that makes it simple to publish magazines, catalogs, newspapers, books, and more online.Easily share your publications and get.Insider info on Alaska North Slope Jobs whos hiring, what theyre hiring for, contact information, plus the latest workrelated gossip from the slope.Get the answers to your questions about our online interactive training courses, including enrollment questions and technical requirements.This is a one day PYA Approved Course as part of the G.U. E. S. T. Training Program.The prerequisite for the course is the Introductory Wine Cocktail Bartending I.I is one. The vast majority of radiation detectors out there and virtually every detector used in general situations fall into one of these two families.So since I wrote about the gas filled detectors last time, this is a good chance to write about the other major family.Heres how the scintillation type detectors work.First, an important point.Most scintillation detectors are only sensitive to one type of radiation.So Na. I detectors will pick up gamma radiation, but not alpha or beta, zinc sulfide Zn.S will only pick up alpha radiation, and so forth.In actuality, there might be some sensitivity to other radiations Na.I, for example, will sometimes pick up high energy betas but you should only use a detector for the type of radiation its designed to pick up.Now, with that out of the way, on to how the things workThe basic principle is the same for every scintillation type detector when radiation strikes the scintillator it causes it to give off photons of visible light thats the scintillation part.These photons pass through the crystal and they strike a thin metal foil called a photocathode when this happens the light enters the second part of the detector, called a photo multiplier tube PMT.When the photon hits the photocathode it causes an electron to be ejected from the photocathode.Just past the photocathode there is a set of metal cups, each with a voltage applied to it typically several hundred to a thousand volts the electron is accelerated by this voltage to a high energy and it strikes the cup with enough energy that it knocks loose a number of other electrons.Each of them, in turn, is accelerated towards the next metal cup, where each of the new electrons knocks loose a number of additional ones by the end of the PMT the initial signal has been multiplied by a factor of a million or so.From there, its up to the instrument manufacturer to figure out how theyre going to use the light thats emitted.As one example, every time a gamma hits the crystal it starts this whole process that culminates in a pulse of electrons arriving at the far end of the detector.The simplest way to deal with this is to simply count the pulses of electrons as they arrive at a counting circuit this is a great way to measure contamination which we normally record in terms of counts per minute or counts per second.We can also use this mode to measure radiation dose rates, but only by assuming that every count carries with it a specific amount of energy.If you remember the posting on gas filled detectors, this is the same way that Geiger counters work and its one of the reasons that Geiger counters arent always accurate for measuring radiation dose rates.Well get back to that in a moment.Oh one other thing to be careful about with an Na.I detector is that the larger crystals say, 2x.So if youre working outside on a blazing hot or bitingly cold day, you probably want to leave the crystal outside, rather than bring it into a much colder or warmer office a few times a day.Something else that we can use Na.I detectors for is identifying specific radionuclides by measuring the energy of each individual gamma that enters the crystal this process is called gamma spectroscopy, or can also be called multi channel analysis and the instrument set up for this purpose is called a gamma spectroscopy device, or a multi channel analyzer abbreviated MCA.The basic principle behind gamma spectroscopy is that every gamma emitting radionuclide emits a gamma ray or a few gammas with very specific energies like a fingerprint and if we can identify the gamma energies precisely enough then we can identify the radionuclides present.For example, cesium 1.Cs 1. 37 gives off a gamma with an energy of 6.V if we analyze a gamma ray spectrum and find a peak with an energy of 6.V then we know that Cs 1.Along the same lines, seeing twin gamma peaks at about 1.Me. V tells us that weve found cobalt 6.Co 6. 0. The problem is in figuring out how much energy is in each gamma photon luckily we can do this with a scintillation detector.When a gamma ray interacts with the Na.I crystal it deposits energy this energy is what causes the photons to be given off.Not only that, but a predictable number of photons are emitted depending on the energy deposited in a sodium iodide crystal, depositing 1 Me.V of energy will cause about 4.We know the number of photons that it takes to eject a single electron from the photocathode, and we know the amount of amplification and the size of the electrical pulse for each electron ejected.So if we can measure the size of the output pulse then we know how much energy was deposited in the crystal and we can know what radionuclide emitted the gamma that we just detected.So these are two uses of Na.I detectors measuring contamination levels in counts per minute or counts per second and identifying radionuclides by measuring gamma energies now for a third, measuring radiation dose rate.Radiation dose rate is a measure of the amount of energy deposited in an object.Youd think that this would be fairly simple with an Na.I detector since the number of electrons coming out of the detector is proportional to the energy deposited in the crystal.But thats not how most manufacturers handle things most of the time they simply count the pulse rate and, as with a Geiger counter, assume that they are all coming from Cs 1.So, for example, the manufacturer might determine that 1.Rhr. This means that Na.I detectors set up to measure dose rate this way have the same limitations as a Geiger counter unless you are measuring exactly what they were calibrated with the dose rates you measure are going to be wrong.Thus, if youre using a scintillation detector to measure radiation dose rate you need to make sure that either youre measuring the same nuclide to which it was calibrated or you need to have a set of correction factors that will let you convert the meter reading to the correct dose rate assuming that you know the nuclide thats actually present.This graphic, which is for a sodium iodide scintillation detector, show that the meter will show only half the actual dose rate if the radiation is from a Co 6.Am 2. 41 in, say, a box of smoke detectors. Satellite Tv For Pc 2011 Works On Win7 Wallpaper . OK so that covers how the Na.I detector works and what it can be used for again, to measure gamma contamination, for nuclide identification, or with caveats to measure radiation dose rate.Now a little on other types of scintillators.Zinc sulfide Zn. S is used to measure alpha contamination.The Zn. S crystals are razor thin only about as thick as a single human hair give or take a little.But since alpha particles cant penetrate very far into any materials the crystals dont need to be any thicker than this.For a number of reasons we dont worry about dose rate from alpha radiation, so the only thing we need to measure is count rate a fairly simple matter of counting pulses.The biggest problem with Zn.S detectors is that they can be fragile remember how thin the crystals are.It can also take a long time to do a proper alpha contamination survey since alphas are so easily shielded and have such a short range in air.But for alpha counting, Zn.S is about as good as it gets, in spite of its limitations.Finally, there are also beta scintillators.Liquid scintillation counting is normally performed in the laboratory using fairly expensive and large machines chances are that you wont have to use one of these unless you work in a laboratory.There are also beta scintillation crystals that you use the same way you use an Na.I detector these tend to be made of plastic called organic scintillators.While not as fragile as Zn.
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