Topic 4. Problems in Radiation Detection and Measurement

1. Topic 4. Problems in Radiation Detection and Measurement Detection Efficiency Problems in the detection and measurement of beta particles Deadtime

2. Detection Efficiency The Definition Geometric Efficiency Intrinsic Efficiency Energy Selective Counting Absorption and Scatter

3. The Definition Emission Rate (assume ? rays per disintegration): ?(?rays/sec)=3.7�104(dis/�Ci.sec) �A(�Ci) � ? (?rays/dis) Detection Efficiency (with R the recorded counting rate): D=R/ ?

4. Detection Efficiency Detection efficiency depends on a number of factors: absorption and scatter (F), geometric efficiency (g), intrinsic efficiency (e) and energy-selective counting (f). It can be expressed as: D=F�g � e �f

5. Geometric Efficiency Inverse square law: I=?/4pr2 where I is the intensity of radiation per unit area and ? is the source emission rate. For point source, geometric efficiency is gp=a/4pr2 where a is the detector surface area and r is the distance from the point source.

7. Geometric Efficiency (small distance r) Assume the detector is close to the source (small r), the area of the detector is S=Or2 where O=2p(1-cos?). The geometric efficiency is then gp=S/4pr2=Or2/4pr2=(1-cos?)/2 When source in contact with the detector ?=90o, gp=1/2 and when source is immersed in the detector material, ?=180o, gp=1.

10. Intrinsic Efficiency Intrinsic efficiency is defined as e=(no. of radiation interacting with detector)/(no. of radiations striking the detector) For ?-ray detector, e={Io-Ioexp[-�l(E)x]}/Io =1-exp[-�l(E)x] where Io is the incoming ?-ray intensity and Ioexp[-�l(E)x] is the ?-ray intensity that pass through the detector without interaction.

11. Effect of ? ray energy and detector thickness on Intrinsic Efficiency For NaI(Tl) detector, the intrinsic efficiency increases with the increase of thickness x and decreases with the increase of the photon energy (x~5cm, for most nuclear medicine energy, e~1) For semiconductor detector, the intrinsic efficiency is also energy dependent (comparison with NaI(Tl) is complicated due to the coupled atomic number and the crystal density)

13. Intrinsic Efficiency (gas filled detectors) For gas filled detectors, the intrinsic efficiency is very small for photons (?, X, e <0.01) but good for particle radiations (a,�, e~1). Detection of ? rays by gas filled detectors is mostly via electrons knocked out from the wall by the ? rays.

14. Energy Selective Counting Not all output signals are counted in energy selective counting Photo-fraction fp is defined as the ratio of the counts in the photo-peak over the signal counts produced by the detector. Photo-fraction depends on ? energy, detector material and detector size. Full spectrum counting provides the maximum possible counting rate (used for single radionuclide with small scattering).

16. Non-uniform Detection Efficiency

17. Detector Profiles

18. Minimum Source-Detector Distances

19. Coincident Detection(Multiple Emission) R1=?1xD1xA R2= ?2xD2xA R12= ?1xD1x ?2xD2xA R=R1+R2-R12

20. Absorption and Scatter (1) Absorption and scatter occur when radionuclide is embedded within some media (tissue for instance). Counting rate is decreased by absorption but may be decreased or increased by scattering depending on the numbers of the scattering from and into the detector. Absorption and scatter are dependent on the ? energy, depth within the medium and if the energy selective counting is used.

22. Absorption and Scatter (2) Absorption and scattering decreases with increase of the ? energy Absorption may predominate if the radionuclide is embedded in greater depth. Scattering is increased initially and reaching a maximum and then decreased with the increase of energy.

23. Determination of Detection Efficiency Use known calibration source Calculation by definition equation D=R/? Take into account the emission frequency if the calibration source is differing from the radionuclide concerned (see next table).

26. Problems in the detection and measurement of beta particles Liquid Scintillation Detectors are generally used for short ranged � particle detection. A survey meter may be used to detect surface contamination by � particles provided it has an entrance window sufficiently thin to permit the � particles to enter the sensitive volume of the detector

28. Gas Filled Detectors in � Assay Sample self absorption (before reaching the detector) and backscattering (from sample and sample holder) are the problems. Self absorption depends on sample thickness and � energy (strong absorption for low energy and thicker samples). Backscattering increases the sample counting rate (20-30%).

31. NaI(Tl) For High Energy � NaI(Tl) detectors can be used to detect higher energy � emitting radionuclides (32P, Emax=1.7 Mev, not 14C, Emax=0.156MeV) by counting the Bremsstrahlung radiation. Greater activities are required (1000 times more than ? emitters) because of the low production efficiency of Bremsstrahlung radiation.

32. Deadtime Cause of Deadtime Mathematical Models Window Fraction Effects Deadtime Correction Methods

33. Causes of Deadtime (1) Deadtime is the time required for the counting system to process an individual event (it is also called pulse resolving time t). Several components (detector, pulse amplifiers, pulse high analyzer, scaler and computer interface) could contribute to the deadtime and the longest component determines the system deadtime.

34. Causes of Deadtime (2) The deadtime of NaI(Tl) or semiconductor detectors are mainly caused by pulse amplifiers (pulse pileup and baseline shift, typical 0.5-5 �sec) GM tubes are mainly caused by detector�s pulse overlap (typical 50-200 �sec). Deadtime results in counting losses (overlap loss or window shift loss).

35. Mathematical Models Counting systems can be classified as paralyzable and non-paralyzable type and most of nuclear medicine systems are paralyzable type. A non-paralyzable system is one for which, if an event occurs during the deadtime t of a preceding event, then the second event is simply ignored. A paralyzable system is one for which each event introduces a deadtime t whether or not that event actually was counted.

37. Nonparalyzable Systems(1) The relationship between observed count rate Ro and true count rate Rt for the nonparalyzable system is given by

38. Nonparalyzable Systems(2) The observed counting rate Ro has a maximum value. For nonparalyzable system, it is given by

40. Paralyzable Systems(1) The relationship between observed count rate and true count rate for the paralyzable system is given by

41. Paralyzable Systems(2) The observed counting rate Ro also has a maximum value. For paralyzable system, it is given by

42. Deadtime Losses Deadtime losses is defined as (Rt-Ro) and the percentage losses (PL) is given by

43. Window Fraction Effects Deadtime losses depend on total counting spectrum. Deadtime per event in the selected window depends on the fraction of the window: ta= t/wf Window fraction effect must be considered in comparing deadtime of different systems using energy selective counting

44. Deadtime Correction Methods True counting rate can be calculated if we know the deadtime and the observed counting rate (either use the equation for nonparalyzed system directly or use graphic or approximation for paralyzed system). Deadtime can be calculated by using the maximum observed counting rates Deadtime can also be measured by using two source method

45. The Two Source Method Two equal sources and three measurements, R1, R12, R2. Equal time should be used in short lived radionuclide to avoid decay correction. The dead times are calculated by

47. Fixed Rate Pulser Method A fixed rate pulser is connected to the preamplifier of the radiation detector. Different input pulse rates (Pt) are generated (pulse hight larger than the photo-peak) and the observed counting rates (Po) are recorded. Real detector system is then connected to the preamplifier and the observed counting rates (Ro) are measured. The true counting rate (Rt) is then calculated by