1. 4/2003 Rev 2 II.3.12 – slide 1 of 47 Session II.3.12 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources Part IIQuantities and Measurements Module 3Principles of Radiation Detection and Measurement Session 12Efficiency, Geometry

2. 4/2003 Rev 2 II.3.12 – slide 2 of 47  We will discuss how to convert measured values so that they represent the actual amount of activity present Overview

3. 4/2003 Rev 2 II.3.12 – slide 3 of 47 Surface Contamination RemovableFixed

4. 4/2003 Rev 2 II.3.12 – slide 4 of 47 Point Source Dispersed Source Surface Contamination

5. 4/2003 Rev 2 II.3.12 – slide 5 of 47 Factors for calculating dpm from cpm where:  IE = intrinsic efficiency of instrument (counts per hit)  GE = geometric efficiency (hits per particle emitted)  Y = yield of radionuclide (particles emitted per disintegration) NOTE: a “hit” implies that a photon or particle enters the detector but it may or may not result in a “count” x x x = c m 1 IE 1 GE1Y d m Surface Contamination

6. 4/2003 Rev 2 II.3.12 – slide 6 of 47 Factors for calculating dpm from cpm x x x = c m 1 IE 1 GE1Y d m c m 111d m c h p d h p c m d m h cdp p h Surface Contamination IE = c/h GE = h/p Y = p/d

7. 4/2003 Rev 2 II.3.12 – slide 7 of 47 For the general case of several different types of radiations: For the simple case of one type of radiation (as presented on the previous slides): c m d m =x (IE x GE x Y) 1 {(IE 1 x GE 1 x Y 1 ) + (IE 2 x GE 2 x Y 2 ) + …} c m d m = x1 Surface Contamination Factors for calculating dpm from cpm

8. 4/2003 Rev 2 II.3.12 – slide 8 of 47 = x x x cm IEGEYdm Surface Contamination Factors for calculating cpm from dpm where again:  IE = intrinsic efficiency of instrument (counts per hit)  GE = geometric efficiency (hits per particle emitted)  Y = yield of radionuclide (particles emitted per disintegration)

9. 4/2003 Rev 2 II.3.12 – slide 9 of 47 = x x x cm IEGEYdm c m d m c hpdhp Surface Contamination Factors for calculating cpm from dpm IE = c/h GE = h/p Y = p/d

10. 4/2003 Rev 2 II.3.12 – slide 10 of 47 = x x x cm IEGEYdm = {(IE 1 x GE 1 x Y 1 ) + (IE 2 x GE 2 x Y 2 ) + …} x cmdm For the general case of several different types of radiations: Factors for calculating cpm from dpm Surface Contamination For the simple case of one type of radiation (as presented on the previous slides):

11. 4/2003 Rev 2 II.3.12 – slide 11 of 47 Geometric Efficiency or 2222 4444 GE = h/p If the instrument is placed on top of the source (e.g., a portable survey instrument), then the GE = 0.5 h/p or less. However, if the detector surrounds the source (e.g., a “well counter”), then the GE = 1 h/p or less.

12. 4/2003 Rev 2 II.3.12 – slide 12 of 47 Geometric Efficiency or 2222 4444 GE = h/p If every particle emitted in the direction of the detector strikes the “sensitive volume” of the detector, then we should have a perfect geometric efficiency of 0.5 or 1.0 depending on whether it is a 2  or 4  geometry. However, some of the particles impinging on the detector may not interact with the sensitive volume (e.g. they are unable to penetrate the detector faceplate or they escape through a small gap such as exists at the entrance of a well counter)

13. 4/2003 Rev 2 II.3.12 – slide 13 of 47 A vendor of survey instruments states: “Beta Efficiency = 35% as a percent of 2  emission rate”  This means that the detector “counts” 35% of all the particles emitted in the upward direction (towards the detector)  Its intrinsic efficiency (IE) is 35% (it counts 35 out of every 100 particles that hit the detector) Example

14. 4/2003 Rev 2 II.3.12 – slide 14 of 47  Remember, the ultimate goal is to detect the amount of surface contamination (Bq or dpm), so you MUST be able to account for ALL of the particles emitted not just those striking the detector  This will give you the total number of disintegrations per minute (dpm) 0.35 x 0.5 = total efficiency = 0.175 c h c p h p Example A vendor of survey instruments states: “Beta Efficiency = 35% as a percent of 2  emission rate”

15. 4/2003 Rev 2 II.3.12 – slide 15 of 47 If the detector is not in contact with the source (i.e., it is not as close as possible to the source), then some of the particles travelling upwards may not hit the detector. In that case, the Geometric Efficiency (GE) has two components: GE = = x h p h u u p where u/p is the fraction of the particles emitted upwards (normally 0.5) and h/u is the fraction of the upward particles that actually hit the detector which could be any fraction from 1 to 0 depending on how close or far the detector is from the source. Example

16. 4/2003 Rev 2 II.3.12 – slide 16 of 47 GE = = x h p h u u p  For example, if an alpha detector is placed on top of an alpha emitting source, the GE (h/p) would equal 0.5 (u/p = 0.5 and h/u = 1) which means that ½ of the particles are emitted upward towards the detector and every alpha particle traveling in that direction struck the detector  But if the same detector were raised about 5 cm above the source, the GE would equal 0. Even though u/p would still be 0.5 (the same number of particles are traveling upward towards the detector), h/u would be 0 since none of the upward particles would reach the detector, they would be stopped by the intervening air gap Example

17. 4/2003 Rev 2 II.3.12 – slide 17 of 47 The surface contamination limits in the United States are given in terms of dpm per 100 cm 2. You are performing a surface contamination survey using an alpha probe which has a scale calibrated in cpm. The sensitive area of the probe is 60 cm 2. Briefly explain what you would do to make your measurement(s) consistent with the limits. ANSWER: You would have to convert cpm to dpm using the process described in the previous slides and then you would have to scale your measurements from 60 cm 2 to 100 cm 2. Sample Problem 1

18. 4/2003 Rev 2 II.3.12 – slide 18 of 47 A technician uses a GM pancake probe to monitor a tabletop for beta/gamma surface contamination. The probe has a sensitive area of 40 cm 2. The technician surveys an area measuring about 400 cm 2 and obtains an average reading of 2,300 cpm. The background is 100 cpm. The instrument has a total efficiency (intrinsic plus geometric) of 10%. What is the contamination level in terms of dpm per 100 cm 2 ? Sample Problem 2

19. 4/2003 Rev 2 II.3.12 – slide 19 of 47 Sample Problem 2 Measurement is 2,300 cpm – 100 cpm = 2,200 cpm x x x = c m 1 IE 1 GE1Y d m IE x GE = 0.1 Assume that Y = 1 x x = cm10.111dm x 10 = cmdm dpm = 10 x 2,200 cpm = 22,000 x 100 cm 2 = 22,000 dpm 40 cm 2 55,000 dpm 100 cm 2

20. 4/2003 Rev 2 II.3.12 – slide 20 of 47 Sample Survey Terms

21. 4/2003 Rev 2 II.3.12 – slide 21 of 47 Sample Survey Frequencies

22. 4/2003 Rev 2 II.3.12 – slide 22 of 47 Reasons for Surveys

23. 4/2003 Rev 2 II.3.12 – slide 23 of 47 Survey Requirements

24. 4/2003 Rev 2 II.3.12 – slide 24 of 47 Radiation Level Surveys vs Contamination Surveys Surveys

25. 4/2003 Rev 2 II.3.12 – slide 25 of 47 Ionization chamber: Measures exposure rate which is what we want GM detector: May measure cpm but we want exposure rate Radiation Level Monitoring 124 124

26. 4/2003 Rev 2 II.3.12 – slide 26 of 47 Intrinsic Efficiency (IE)  For contamination monitoring we need to relate the number detected to the number emitted from the source because that relates to the activity on the surface which is what we want to know  For radiation level monitoring we are interested in the “exposure rate” which is a function of how many free electrons are produced in our detector which is dependent on how many photons hit our detector but independent of how many are emitted from the source

27. 4/2003 Rev 2 II.3.12 – slide 27 of 47 cmincsec1 area of detector (cm 2 ) x = Converting cpm to mR/hr The simplest way to convert from cpm to mR/hr using a GM detector is to use an instrument with a dual scale. In this image, 1 mR/hr = 1,200 cpm c cm 2 - sec 1IE c h h x =

28. 4/2003 Rev 2 II.3.12 – slide 28 of 47 This quantity can be converted to mR/hr by using a photon fluence graph which tells us how many photons per cm 2 per sec is required to produce one R/hr photons cm 2 - sec Converting cpm to mR/hr

29. 4/2003 Rev 2 II.3.12 – slide 29 of 47 Calibration of a NaI Detector If an instrument is calibrated in a fixed geometry and then used in that same geometry, it can be “taught” to provide the information desired. For example, here is a 13 cm diameter sodium iodide (NaI) detector called a FIDLER (Field Instrument for the Detection of Low Energy Radiation)

30. 4/2003 Rev 2 II.3.12 – slide 30 of 47 Calibration of a NaI Detector If we calibrate this instrument properly, although the meter indicates cpm, we can instantly convert the results to Bq/m 2 As this figure shows, the detector is placed at a fixed distance from the surface (30 cm) both during calibration and during use

31. 4/2003 Rev 2 II.3.12 – slide 31 of 47 Point Sensitivity: Point Sensitivity: If we place our detector at some height (h) and place a source of radiation Q directly under it, we can determine the point source sensitivity (S p ) of the detector which is just the net counts (N) divided by the activity of the source (Q) which yields: h Sp =Sp =Sp =Sp = =NQcpmBq The point source sensitivity can be used to convert any reading from cpm to Bq provided the height (h) doesn’t change and the radionuclide is the same. Calibration of a NaI Detector

32. 4/2003 Rev 2 II.3.12 – slide 32 of 47 Area Sensitivity: First draw some concentric rings under a detector with the source in the center. If we now move the source away from the center to one of the rings, we can determine the sensitivity of the detector to radiation off-center. Calibration of a NaI Detector

33. 4/2003 Rev 2 II.3.12 – slide 33 of 47 We can move the source along the ring and make a measurement at each point but we should get the same reading each time since the detector is in the center of the ring. The sensitivity would be the total counts from the ring divided by the total activity in the ring which is the same as a single count divided by a single source on the ring. r Calibration of a NaI Detector

34. 4/2003 Rev 2 II.3.12 – slide 34 of 47 So we only have to make one measurement for each ring to determine the sensitivity of the detector to a source at that distance off center. h r r Calibration of a NaI Detector

35. 4/2003 Rev 2 II.3.12 – slide 35 of 47 Area Sensitivity: The area of one of the rings is equal to the area of the circle with radius (r +  r) minus the area of the circle with radius r Calibration of a NaI Detector rrrr r r+  r

36. 4/2003 Rev 2 II.3.12 – slide 36 of 47 where  r is the width of the ring however, (  r) 2 can be ignored since it is a small perturbation which leaves 2  r  r where  r is a fixed width and r varies depending on which ring we are looking at however, (  r) 2 can be ignored since it is a small perturbation which leaves 2  r  r where  r is a fixed width and r varies depending on which ring we are looking at.  (r +  r) 2 - r2r2r2r2 which reduces to: 2r2r2r2r  r + (  r) 2  r2 + r2 + r2 + r2 + 2  r  r + (  r) 2 -  r 2 Expanding yields: r+  r r Calibration of a NaI Detector

37. 4/2003 Rev 2 II.3.12 – slide 37 of 47 Note that 2  r  r is the same as  D  r or C  r where D is the diameter of the circle and C is the circumference. C  r is just the circumference times the width which is the area of the ring. To obtain the sensitivity of the detector to the entire area of contamination we would have to integrate over each ring as the radius goes to infinity. Actually, after the radius gets to a certain point, the source will be so far from the detector that the detector will no longer see any of the radiation emitted. Calibration of a NaI Detector

38. 4/2003 Rev 2 II.3.12 – slide 38 of 47 An integral is the same as a summation so for practical calibration we can make measurements at discrete points and sum the results to obtain the area sensitivity of the detector. For each ring with a new radius (r) we will obtain a new net result (N) in cpm so that the summation is: (2  r 1  r)N 1 Q (2  r 2  r)N 2 Q (2  r 3  r)N 3 Q +++ …. Calibration of a NaI Detector

39. 4/2003 Rev 2 II.3.12 – slide 39 of 47 However, 2, ,  r and Q are constant so they can be taken out of the summation and we are left with: where the only variables are r which is the radius of the circle (the distance from the source to the center of the circles under the detector) and N which is the net counts (cpm above background) obtained with the source at that point where the only variables are r which is the radius of the circle (the distance from the source to the center of the circles under the detector) and N which is the net counts (cpm above background) obtained with the source at that point. 2  r Q  riNi riNi riNi riNi Calibration of a NaI Detector

40. 4/2003 Rev 2 II.3.12 – slide 40 of 47 If the points of measurement (r) are chosen to be 0, 5, 15, 25, 35 … 105 cm, then  r is 10 cm and the equation becomes: (assuming Q is in Bq and r and  r are in cm) Calibration of a NaI Detector 20  Q  r i N i = area sensitivity = = cpm cm 2 cpm - cm 2 Bq Bq

41. 4/2003 Rev 2 II.3.12 – slide 41 of 47 If you prefer m 2 instead of cm 2 then r and  r (both in cm) can be converted to meters by multiplying each by (1 m/100 cm). We end up with 20/(100 x 100) = 2 x 10 -3. The equation then becomes: Calibration of a NaI Detector 2 x 10 -3  Q  r i N i = area sensitivity = cpm m2 m2 m2 m2 Bq

42. 4/2003 Rev 2 II.3.12 – slide 42 of 47 standard = 2.2 x 10 5 Bq or dpm standard = 4,440 cpm (above background) efficiency = (4,440 cpm)/(2.2 x 10 5 dpm) = 0.02 c/d = 2% contamination = 150 cpm/0.02 c/d = 7,500 dpm or Bq Sample Problem sampleholder end window GM detector moveableshelf background = 50 cpm sample = 200 cpm sample = 200 cpm net sample = net sample = 200 cpm - 50 cpm = 150 cpm 200 cpm - 50 cpm = 150 cpm

43. 4/2003 Rev 2 II.3.12 – slide 43 of 47 To evaluate the dpm on a swipe do the following: Contamination Summary cpm for sample - cpm for background efficiency of the system (counts/disintigration) cpm for a known standard - cpm for background dpm of the known standard To determine the efficiency of the system do the following:

44. 4/2003 Rev 2 II.3.12 – slide 44 of 47  Use appropriate instrument for type of radiation  Stabilize instrument, use check source  Turn instrument on in background area, move up range as necessary  Survey slowly to accommodate slow response time (2 to 5 cm per second), close to surface  Use sound, the more senses, the better  Compare readings to licensee’s surveys  Record all information Good Survey Practices

45. 4/2003 Rev 2 II.3.12 – slide 45 of 47  Changing geometry  Partial detector irradiation (small beams)  Extreme environmental conditions  Varying energies and mixtures  Composition of contaminated surfaces  Variances in calibrations (sources, geometry, scatter, electronic)  Pulse versus continuous radiation  Dose vs Exposure and counts per minute (cpm) versus dpm Variable Survey Conditions

46. 4/2003 Rev 2 II.3.12 – slide 46 of 47  Use the correct wipe materials  Wet or dry (efficiency versus self shielding)  Transportation losses  Efficiency of wipe  Geometry, detector, window, background, calibration  Interpretation of results  Your measurement techniques vs licensee’s - Let them demonstrate Variables of Removable Wipes

47. 4/2003 Rev 2 II.3.12 – slide 47 of 47 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th Edition, 1999 update), Wiley, New York (1999)  International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)