University of Cape Town Department of Physics Course III laboratory Gamma ray coincidence and angular correlation Introduction Medical imaging based on positron emission tomography (PET) continues to have wide-ranging clinical impact, particularly in cancer diagnosis and management, cardiology and neurology. At the basis of PET is the decay of particular isotopes which emit a positron, the anti-particle of the electron, first postulated by Dirac in 1928, and observed by Anderson in 1932. A positron produced in a nuclear decay will rapidly annihilate with an electron, resulting in a pair of 511 kev gamma rays which are emitted almost back-to-back. If both of these gamma rays are detected at two points in space, then the origin of these gamma rays must have occurred somewhere along this straight line. A three dimensional image may be constructed from a collection of such coincidence events, measured at many angles, using the analytical methods of tomography. Positron-emitting radionuclides are nowadays easily produced using particle accelerators, including ithemba LABS. The positron emission particle tracking group at UCT have a high resolution ring geometry PET scanner which is used for particle tracking for studies of particulate flow. 1
The experiment In this experiment pairs of gamma rays emitted within a very short time (<< 1 µs) of each other are examined. The to be investigated is mounted at the centre of a circular platform. Two NaI(Tl) detectors are mounted on tracks so that their radical distances from the can be varied. The detectors can also rotate on the platform so that their angular separation may be varied. In this experiment you will use the pair of NaI(Tl) scintillation detectors to study gamma rays detected in time-coincidence, in order to (a) measure the angular correlation P(θ) for 0.511 MeV photon pairs emitted from a 22 Na ; (b) investigate coincident gamma spectra measured for 22 Na and 60 Co s; (c) determine the absolute efficiency of one of the NaI crystals for detecting 0.511 MeV gamma rays; and (d) measure the absolute strength of the 60 Co. Set the experiment up as shown below: Detector 1 Detector 2 or ADC/MCA Place the two detectors equidistant from a 60 Co and at 180 angular separation. Set the such that the most prominent -ray signals observed at the output of the s are about 3 volts. Record pulse height spectra on the MCA for each detector. Adjust the gains on the s until the two photopeaks appear at more or less at the centre of the ADC range. Replace the 60 Co with a 22 Na and record singles pulse height spectra separately for each detector. These singles 60 Co and 22 Na spectra can be used to determine an energy scale for each detector. You should not make further adjustments to the and gains. 2
(a) Angular correlation measurements θ Detector 1 Detector 2 Coincidence unit Scaler N 1 Scaler N co Scaler N 2 The angular correlation function P(θ) of two gamma rays, 1 and 2 may be defined as the relative probability of 1 and 2 being emitted at relative angle θ. For 0.511 MeV photons from 22 Na we expect P(θ) to be strongly peaked at θ = 180. Why? Set up the electronics as shown below. Set the pulse height windows on each SCA as wide as possible. Use the scalers to count singles counts from each detector (N 1 and N 2) and coincidences (N co). Vary θ to measure P(θ). Keep factors such as angular resolution in mind when planning measurements and interpreting results. Take readings at 1 intervals for θ near 180. Correct your data for accidental coincidences. How do your results change if you set windows on the SCAs to select only the photopeak associated with the 0.511 annihilation radiation? 3
(b) Coincident gamma spectra θ Detector 1 Detector 2 gate ADC/MCA The singles gamma spectra from 22 Na and 60 Co will be familiar from your introductory gamma ray spectroscopy experiment. Coincidence measurements provide additional information about these spectra and also about the nuclear transitions responsible for the -rays. A pulse height window should be set on the output from detector 1. Pulses from detector 2 are then filtered (or gated ) so as to select for measurement (on the MCA) only those in coincidence with the window signal of detector 1. Explore coincidence spectra measured with detector 2 for 22 Na and 60 Co (by setting the window on the SCA for detector 1 on either the 0.5 MeV photopeak of 22 Na or the 1.33 MeV photopeak of 60 Co). Explain your spectra carefully. It may make sense to measure the 60 Co spectrum over night. (c) Absolute efficiency determination Your result from (a) above can provide a basis for a method of determining 1 (0.5), the absolute efficiency of detector 1 for detecting a 0.511 MeV photon moving axially through the detector. Work out the method for yourself. The measurements you require are of N 1, N 2 and N co (observed at θ = 180.) Measure using d2 3d1 and for a time interval sufficient to ensure an uncertainty of less than 10% on N co. 4
(d) The absolute strength of the 60 Co The absolute strength of the 60 Co may also be calculated. Let the activity of the be D (total number of disintegrations per second). 1 Then the count rate from detector 1 is R1 D1 4. where 1is the efficiency of detector 1 and 1 is the solid angle that the crystal subtends at the. 2 In the same way, the count rate from detector 2 is R2 D 2 4. For every 1.33 MeV gamma ray a 1.17 MeV gamma ray is emitted. If detector 1 detects a gamma ray 2 then the probability that detector 2 will detect the coincident gamma ray is 2 4 slight anisotropy). (neglecting the 2 Hence if the rate detected by detector is R 1 then the coincident rate will be Rco R 1 2 4. RR 1 2 Substituting for 22we find Rc. D RR 1 2 Therefore, D. R co Use the calibrated for this experiment and compare your calculated absolute activity with what it should be. (The half life of 60 Co is 5.26 years and 1 curie = 3.7 10 10 disintegrations per second.) ab/24aug2014 5