Alpha Particle Calibrations

1 Page 1 NIST Measurement Services: Alpha-Particle Calibrations (2004) J. M. R. Hutchinson NIST Special Publication 250–5a National Institute of Standards and Technology Technology Administration, U.S. Department of Commerce Open at page → 2 Page 2 NIST Special Publication 250–5a NIST MEASUREMENT SERVICES: Alpha-Particle Calibrations (2004) J. M. R. Hutchinson Ionizing Radiation Division Physics Laboratory National Institute of Standards and Technology 100 Bureau D... Open at page → 3 Page 3 Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or... Open at page → 4 Page 4 iii Table of Contents Abstract................................................................................................................ iv List of Figures.............................................................. Open at page → 5 Page 5 iv ABSTRACT This document describes the alpha-particle calibration services offered by the Radioactivity Group of the National Institute of Standards and Technology (NIST) (Scheduled Calibrations; 43030C, 43040C, and 430... Open at page → 6 Page 6 v List of Figures 1. Schematic of the small 2πα PC 2. Electrical hook-up for all counters 3. Photo of the small 2πα 4. Relative count rate as a function of counter voltage for the small 2πα PC 5. Relative count rate as a... Open at page → 8 Page 8 1 1. Introduction Alpha particles emitted from radioactive sources are measured at the National Institute for Standards and Technology (NIST) in three systems, the 2π steradian, α-proportional counting system (denoted sm... Open at page → 9 Page 9 2 Calibrations are checked by simultaneously calibrating a previously- calibrated standard source. Typical calibration reports for 2πα-emission rate measurement and 0.8πα-particle emission rate are given in Appendices Ei... Open at page → 10 Page 10 3 3. Design Philosophy and Theory * The proportional counters [2] are used because they record alpha particles with little interference from emitted beta particles or gamma rays. In proportional counters, all alpha parti... Open at page → 11 Page 11 4 12 CENTIMETER DIAMETER PROP ORTIONAL COUNTER CHAMBER (small 2πα PC) Fig. 1. Schematic diagram of the small area 2πα PC Open at page → 12 Page 12 5 Fig. 2. Hook-up of electronic components of all three counters This counter has been in use constantly since that time and is still much used today. Description of the characteristics and usage of such detectors follow... Open at page → 13 Page 13 6 Extrapolation to Zero Pulse Height - Regardless of the type of detection system used, an accurate determination of the total alpha-particle count rate requires that the alpha-particle pulse-height spectrum be recorded... Open at page → 14 Page 14 7 Fig.5. Relative count rate versus position for an 241 Am point source in the small 2πα PC (see Fig. 1). The counter voltage was 1400V. The counting gas was P-10 at atmospheric pressure. The solid line is the count rate... Open at page → 15 Page 15 8 2. It provides a medium in which the free electrons so formed can be accelerated in the presence of an electric field to produce additional ionization (that is, electron multiplication and hence amplification or "gas g... Open at page → 16 Page 16 9 Fig.7. Photograph of the large area counter. The functioning of the large 2πα PC is virtually identical with that of the small 2πα PC. All above descriptions apply. 3.3 0.8πα Scintillation Counter The 0.8πα SC, the so-... Open at page → 17 Page 17 10 the order of 0.1 percent. This detector should not be used with sources with diameters greater than 1.6 cm. However, since the efficiency is approximately 40 percent of that of the 2π counters, maximum source activiti... Open at page → 18 Page 18 11 Fig. 9. Photograph of the 0.8πα SC. Open at page → 19 Page 19 12 Fig. 10. The pulse-height spectrum from a sample of 239 Pu in the 0.8πα SC showing the extrapolation to zero pulse. In about 1960 an experimental investigation of the energy and angular distributions of the emitted an... Open at page → 20 Page 20 13 Fig. 11. Count rate versus angle of emission for alpha particles emitted by 210 Po that had been deposited on the surfaces of platinum and monel disks. (0 is perpendicular to the surface of the disk.) See Ref. 8 for c... Open at page → 21 Page 21 14 [9,10], but not all [7,11], counters of this type are operated with the chamber under vacuum and with a solid scintillator as the alpha-particle detector. A question that always arises is, "What is the intrinsic detec... Open at page → 22 Page 22 15 If one is counting a source that emits only alpha particles, the a/β response ratio is not of significance. Mechanical and chemical properties would be the basis on which to select a scintillator. But if the source al... Open at page → 23 Page 23 16 Without the inside baffle, the source geometrical factor would be sensitive to vertical positioning of the source. As explained by Robinson [9], the three dimensional central baffle is so designed that changes in the... Open at page → 24 Page 24 17 Five collection windows are set covering the lower end of the spectrum for the purpose of obtaining an extrapolation in the case where the total number of counts into 2π are required. The extrapolation is based on the... Open at page → 25 Page 25 18 6. Step-by-Procedures 6.1 Preliminary • Customer contact - give specifications for physical dimensions and activity limits, emphasize that source container must protect active area from contact with anything within th... Open at page → 26 Page 26 19 6.3 Set-up Procedure for All Counters • Introduce source into the counter. • Turn on the High Voltage and other electronics. • Turn on the air pressure valve Flush P-10 gas through the system until the pulses recorded... Open at page → 27 Page 27 20 Data Reduction When the data collection is complete, bring up file called SPECTINC, which sets 6 regions to be used in an extrapolation procedure which gives the integrated total count-rate. Typically the regions woul... Open at page → 28 Page 28 21 • Turn on high voltage (1150 volts) at the start of the flushing. • Reduce flow to approximately 1.5 cubic feet per hour. • Use the following settings for theTenelec TC 247 Dual Amplifier –coarse gain of 200 and fine... Open at page → 29 Page 29 22 To Shut Down • Turn off electronics ( High Voltage and NIM BIN). • Turn of gas (close black knob, main valve, and release the diaphragm control) • Turn off air-pressure valve. • Take smears of counter and immediate ar... Open at page → 30 Page 30 23 Edit file to establish set-up parameters including: - number of sets of measurements(spectra) - elapsed live times - sample description -file name -counter description. • Collect spectral data (hit OPEN). • Hand recor... Open at page → 31 Page 31 24 7. Determination of Uncertainties The corrections and uncertainty components considered are given in Table 3. Table 3. Listing of count rate correction factors and typical 1σ uncertainties for 2π and 0.8π measurements... Open at page → 32 Page 32 25 Although this table lists typical uncertainties, one could, by using these numbers, derive typical total uncertainties for the various measurements by combining the relevant uncertainty components in quadrature and mu... Open at page → 33 Page 33 26 7.4. Scattering off Counter Wall: This effect does not exist in the 2πα PC's counter because alpha particles do not reach the wall. The dimensions of the 0.8π counter are such that very rare large-angle scattering wou... Open at page → 34 Page 34 27 Records maintained are a customer log sheet including customer name, date received, kind of source, source number and identification including Radioactive Source (RS) number and test number, date calibrated, date retu... Open at page → 35 Page 35 28 8. Walker, D.H. "an Experimental Study of the Backscattering of 5.3-MeV Alpha Particles from Platinum and Monel Metal", Int. J. Appl. Radiat. Isotopes, 16, 183 (1965). 9. Robinson, H.P. in Metrology of Radionuclides,... Open at page → 36 Page 36 29 10. APPENDICES Appendix—A Open at page → 37 Page 37 30 Open at page → 38 Page 38 31 Open at page → 39 Page 39 32 Open at page → 40 Page 40 33 Open at page → 41 Page 41 34 Open at page → 42 Page 42 35 Appendix—B Open at page → 43 Page 43 36 Open at page → 44 Page 44 37 Open at page → 45 Page 45 38 Appendix—C Fig. 12. Counting yields, Y(o), as a function of alpha-particle energy, for a plane source at the surface (depth x=o) of a backing of atomic number Z. Open at page → 46 Page 46 39 Appendix—D Fig. 13. Redaction ratio Y av (x0)/Y(o) for uniform layer sources extending to depth x0 below the surface of the backing a. for backings with Z=4 and 13 b. for backings with Z=26 and 78. Open at page → 47 Page 47 40 Appendix—D (continued) Fig. 13b Open at page → 48 Page 48 41 APPENDIX--Ei Open at page → 49 Page 49 42 NOTES (1) The total number of alpha particles counted per second emitted into a 2 π-steradian geometry (including those scattered). (2) The overall uncertainty is two times the value found from combining quadratically... Open at page → 50 Page 50 43 APPENDIX--Eii Open at page → 51 Page 51 44 NOTES (1) The total number of alpha particles counted per second emitted into a 2π- steradian geometry (including those scattered). (2) The overall uncertainty is two times the value found from combining quadratically... Open at page → 52 Page 52 45 APPENDIX--Eiii Open at page → 53 Page 53 46 NOTES (1) The overall uncertainty is two times the value found from combining quadratically the standard deviations of the mean, or approximations thereof, for the following: a) one standard deviation of the mean of f... Open at page →