X-ray flux of 7 times 10^-13 and effective energy of 40 keV is allowed to pass t
ID: 1530569 • Letter: X
Question
X-ray flux of 7 times 10^-13 and effective energy of 40 keV is allowed to pass through the metal window of a fluoroscopy intensifier. The window allows 82 % of the radiation which fell on the first phosphor (CsI: Na) whose conversion efficiency is 45 %. Each photon produced on the phosphor has energy of 2 eV Only 6 % of the photons produce photoelectrons on the photocathode each of energy 1.4 eV. It the photoelectrons are accelerated through a potential difference of 40 kV and on the average each electron produces 0.10 photons on the second phosphor then find the Flux Gain of this fluoroscopy system. a) One Fluoroscopy device uses material A with work function 3.6 eV, as the Photocathode. A second Fluoroscope uses material B with work function 13 eV, as the photocathode. All other properties of the two Fluoroscopes are the same. Which Fluoroscope you'll prefer and why? (b) What is the job of Thallium (T1) in a T1 activated Nal phosphor Explain in detail.Explanation / Answer
5. Answer:
(a) Fluoroscopy is an imaging technique that uses a high intensity beam like X-rays to get the real time moving (dynamic) image of the internal mechanisms of a sample. In this process, we let the incident high intensity beams to penetrate through the sample exceeding its work function to get information about the interior of the sample, like used in medical systems, and security check-ups.
Now, to get better image resolution of the inside, the work function of the material should be less, which makes the incident beams penetrate more deep into it. In this case, we have two materials; A with work function 3.6 eV, and B with work function 3.9 eV. So, material A as photocathode would be the best choice than B, as it will allow less intensity beams to penetrate more into the sample than the later one.
Hence, material A is prefered than B.
(b) The Thallium activated sodium iodide (NaI) is primarily used in scintillation detectors for the detection of chargeless gamma-rays. For this method we need the material to have high intensity detection capabilities. In TI activated NaI phosphors, the activated atoms of Thallium are mainly impurities that are there inside NaI, grown to the crystal to control the spectrum of the light emitted, and also to enhance its intensity. An ideal phosphor has a good absorbance for X-rays, and produce a visible fluoroscence spectrum, and also gives equal pulse heights for every quantum of X-ray incident on the surface. A Tl activated NaI meets these requirements, as it have much better decay times, and also has a high absorbance for the characterestic X-rays.
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