Oslo Cyclotron Laboratory

Transcription

1 Oslo Cyclotron Laboratory Laboratory Excercise in FYS Experimental Methods in Physics Morten A. Salvesen University of Oslo, Oslo, Norway to

2 Contents 1 Abstract 1 2 Oslo Cyclotrone Laboratory 1 3 Singles NaI- and Ge-detectors Calibration Pair Production - Single and Double Escape Unfolding Coincidence Basic Coincidence Technique TDC Calibration NaI Eciency at 511keV and 1275keV Multiplicity Summary 13 6 Appendix Partial Decay Schemes

3 1 Abstract Over the course of 9 weeks spent at the Oslo Cyclotrone Lab working with the CACTUS - a sum of 29 detectors - measuring various radioactive sources, we were to gain an introductory understanding in the use of detectors, their advantages and disadvantages; the use of programs/software; and dierent methods used in experimental nuclear physics. 2 Oslo Cyclotrone Laboratory Built in 1979 the Oslo Cyclotron Laboratory (OCL) has since then formed the basis for experimental nuclear physics in Norway. Outtted with the MC-35 Scanditronix cyclotron it's the only accelerator in Norway for ionized atoms, and is for this reason used in various elds of research and applications. The main focus at OCL is investigation of level densities and radiative strength functions; quantities that are important for understanding of thermodynamic and electromagnetic properties of atomic nucleus, as well as for understanding stellar evolution and accelerator-driven transmutation of nuclear waste. The cyclotron at OCL is also used in medicine for production of radioactive isotopes. Particularly it's been used by the Norwegian Radium Hospital in Oslo for the production of 18 F used in positron emission tomography (PET). Production of 18 F has since been taken over by the Radium Hospital itself, and the production of radioactive isotopes these days is mainly for research purposes. Figure 2 shows the layout of the OCL. The MC-35 cyclotron is able to accelerate protons to a velocity of km/s - 28% of lightspeed, which gives a relativistic mass increase of 4%. Table 1 shows the four available types of particle beams from the cyclotron. Although the MC-35 is the main attraction at the OCL, our studies where only concerned with the CACTUS, g 1, which consists of 26 5x5 NaI detectors and one Ge detector, and the use of software used with the CACTUS. Particle type Energy (MeV) Beam intensity (ua) Proton Deuteron He He Table 1: Available MC-35 cyclotron beams 1

4 Figure 1: The OCL CACTUS: 26 NaI and one Ge detectors Figure 2: General layout of the OCL 3 Singles 3.1 NaI- and Ge-detectors The OCL CACTUS utlizies NaI scintillation detectors and Ge semiconduction detectors. Scintillators work by absorbing high energy electromagnetic radiation which causes the detector to emit (uoresce) photons at visible or near visible light energies, thus releasing the previously absorbed energy, which is detected 2

5 and measured by a photomultiplier tube (PMT). In semiconductor detectors the radiation is measured by looking at the amount of charge carriers that are set free in the detector by the absorbed radiation. The amount of energy required to create an electron-hole pair is known, and is independent of incoming radiation, so measuring the amount of electron-hole pairs gives the energy of the incident radiation. The energy that is required for electron-hole pairs is fairly low, which leads to a higher energy resolution in the Ge semiconduction detectors than in the NaI scintillator. By resolution is meant the ability of the detector to separate gamma rays of dierent energy. It's most common to express the resolution in terms of the Full Width at Half Maximum (FWHM) value; the width of the gamma ray peak at half of it's highest point. The relative resolution is also sometimes used; the FWHM divided by the energy of the gamma ray. As the resolution of detectors varies for dierent energies it's usual to give a reference energy. See gure 3 for a spectrum of a 137 Cs source taken with a Ge detector. Photons of energy 662keV are released as 137 Cs decays to 137 Ba, this is represented by the peak in the spectrum. The FWHM was measured to 4.2keV, which gives a relative resolution of 0.63% at 662keV. In comparison one of our NaI detectors gave a relative resolution of 9.60% at the same energy. Figure 3: Counts vs energy (kev) of 137 Cs measured with Ge detector Earlier the problem with Ge detectors has been the relatively small size, which gives a lower eciency than NaI detectors which have generally been larger. Eciency is the probability that emitted gamma rays will interact with the detector. The eciency of a scintillator is dependent on the relatively slow uorescence decay time of the substance used as detector, whereas in semiconductors it's governed by the fast travel time of the electrons/holes. Techniques 3

6 for growing Ge crystals have improved over the years, so that todays Ge detectors can be made at equal or larger size than the available NaI detectors. The Ge detectors however have a much higher cost than NaI detectors, and larger size consequently gives a very high price. Another disadvantage to the Ge detectors is the requirement that they be cooled down to about liquid nitrogen temperatures (77K) to work. 3.2 Calibration The detectors used all had to be calibrated for the software we used for our measurements, so precise information could be gleaned from the results given by the detectors. In addition, calibrating all the NaI detectors allows us to use all the 26 detectors as one large detector. Calibration for the Ge detector was done by measuring gamma rays with known energy, from 40 K and 228 Th, which have peaks at keV and keV respectively. These isotopes are found in surrounding walls and similar in the OCL, and this background radiation is cause of unexpected peaks when doing experiments. A 137 Cs (662keV) source and 40 K background radiation was used for calibration of the NaI detectors. Figure 4 shows the spectrum from the Ge detector. The peaks are found at channels and corresponding to keV and keV. This gives a calibration factor of keV/ch, and an energy of keV at ch0. This calibration isn't perfect, as the spectrum can't properly be dened by a linear function such as this. Figure 4: Spectrum gained from measuring background radiation from 40 K and 228 Th Calibrating the NaI detectors was done by measuring gamma rays from 137 Cs 4

7 and 40 K, gure 5. The peaks from each is 662keV and keV which gives approximately an 800keV dierence. We wanted the NaI detectors calibrated with 10keV/ch; giving 80ch between the two peaks. This was acomplished by changing the voltage on the PMTs for each detector. In addition, all detectors should be aligned, by making ch0 equal 0keV, thus giving the 137 Cs peak at ch66 for all detectors; gure 6. Recording radiation from 137 Cs using all the NaI detectors as one then gave us a FWHM of 5.9keV, and the peak at ch65.45, giving us a relative resolution of 9.01% at 662keV. Figure 5: Matrix for selection of NaI detectors. 662keV and keV peaks at red/white and orange concentration respectively. Figure 6: Matrix for calibrated and aligned NaI detectors. 3.3 Pair Production - Single and Double Escape It's important to note that not all interactions with the detectors create peaks at the desired energies, as can be seen by the blurring and the non-zero peaks and background peaks in our spectra. The distinct energy peaks are full energy absorption peaks due to photoelectric eect. Background radiation from elements in our surroundings also cause peaks such as those to appear in our spectra, as we saw in section 3.2. There's also counts of photons of most energies up to the maximum full energy peak of the source due to compton scattering. And we have peaks due to pair production. Pair production occurs when a photon interacts with a nucleus inside the detector and creates an electron-positron pair. Electron-positron annihilation is then common, where the particles annihilate to produce two (or more, but this is less likely) photons, with energy 511keV (rest energy of electron/positron). If one of the 511keV photons escapes from the detector, you'll get the single escape peak, which will have an energy 511keV lower than the expected photoelectric peak, likewise if both photons escape the energy will be 1022keV lower. Taking the spectrum from the Ge detector in section 3.2, gure 4, and enhancing the area between ch1900 to ch2500, gure 7, we see a peak at ch2003, 5

8 which equals an energy of 2105keV. This is due to single escape from pair production of the 228 Th keV gamma ray. Double escape is rarer and we were not able to detect any satisfactory peak at the desired energy; the counts registered could just as well be due to noise. Figure 7: Single escape at ch2003 (2105keV) due to pair production from 228 Th keV gamma ray interaction in Ge detector. 3.4 Unfolding As we've seen in our spectra so far, there's a lot of unwanted noise: We have the full energy photoelectric peaks, the single- and double escape peaks, as well as the Compton background, and more. Therefore we need methods that correct our spectra to better represent the full energy spectra we want. One such method is unfolding of a broad energy distributed spectrum into a true full energry spectrum. We will use a folding iteration method [2] to unfold a background reduced spectrum of 152 Eu. The background reduced spectrum, gure 8, was made by running the NaI detectors for equal amounts of time rst with the 152 Eu and then without. Then the background spectrum was subtracted from the original 152 Eu spectrum. The reduced spectrum was then used to create a response matrix, gure 9, required for the folding operation. 6

9 Figure 8: Spectrum of 152 Eu with background radiation subtracted. Figure 9: Response matrix created from bacground reduced 152 Eu spectrum, g 8. In the folding iteration method we dene R i,j as response in channel i when a photon with energy equalling channel j interacts with a detector. Normalization of each response function is then i R i,j = 1, and so folding is given by f = Ru, where f is the folded and u is the unfolded spectra. The folding iteration method then follows four steps: We use u 0 = r as the rst trial function for the unfolded spectrum u 0, where r is the observed spectrum. Then the rst folded spectrum is calculated: f 0 = Ru 0 Then we use the dierence spectrum r f 0 and the original trial function, u 0, to nd the next trial function: u 1 = u 0 + (r f 0 ) We fold and get a new f 1 which is then used to obtain the next trial function: u 2 = u 1 + (r f 1 ) And so on until the folded spectrum f i, where i is the iteration index, is equal to r. Using this method on our reduced 152 Eu spectrum gives us gure 10, which now looks more like a full energy spectrum. Taking a look at the peaks at ch32, ch77, ch96 and ch141, we see that these correspond to the expected full energy peaks of 152 Eu at energies 344.3keV, 778.9keV, keV and keV respectively. The spectrum isn't entirely correct however, as can be seen by the lower than zero parts of the graph. Uncertainties occur due to our assumption in our calculations that there are only 8 full energy peaks, where there are in fact more than this. There's also bound to be some systematic error throughout the process. 7

10 Figure 10: 152 Eu spectrum unfolded to closer resemblance to true full energy spectrum. 4 Coincidence 4.1 Basic Coincidence Technique Often with decay of nucleus two or more photons will be emitted at the same time, f.ex. when an unstable nucleus is created and immediately decays and emits several photons, or, as we shall see with 22 Na; two photons can be produced when a product of the original decay annihilates inside the source. For incidents like these we need measurement methods to separate them from simple single photon occurences; coincidence techniques. Figure 11 shows a simple system for measuring coincidence. The system works by recording all signals that overlap (within a minimum timeframe) as coincidence events. We use the Ge detector as the starter for the timeframe, and the NaI detectors as stop. The curled T in the diagram represents a time delay; as the photon emissions are simultaneous (or very close to) we don't want the stop detector to register before start has occured. 8

11 Figure 11: A diagram for a simple coincidence measurement system. Measuring a 60 Co source and recording both singles and coincidence gave an interesting observation; the NaI detectors closest to the Ge detector had a much higher count of coincidence events than the other NaI detectors which had a more uniform spread. This is most likely due to an eect called cross-talk, where a gamma ray is created by interaction between the incident gamma ray and the Ge detector, the new gamma ray then escapes the Ge detector and interacts with one of the nearby NaI detectors, causing a coincidence event. This also works for NaI to Ge or even NaI to NaI detector. Thus we decided to use six non-neighbouring NaI detectors, as these oered results closer to a true full energy spectrum. Another source of accidental coincidence is random coincidence, where background events arrive within the resolving time of the coincidence circuit. Figure 12 shows singles and coincidence for the 60 Co. The singles spectrum contains three peaks; the 1.17MeV and 1.33MeV from 60 Co, as well as the 1460keV peak from background 40 K. Since the 40 K peak is a single incident it won't be registered as a coincidence event (unless an accidental coincidence happens). If we assume that NC Ge(E γ 1 ) = Ω NaI (E γ2 )NS Ge(E γ 1 ); coincidence count rate for the Ge detector equals the eectiveness of the NaI detector multiplied with the singels count rate of the Ge detector, when the source has gamma multiplicity M γ = 2, we can use this to nd the solid angle for a typical NaI detector (or the combined solid angle for six typical NaI detectors in our case). And since all the NaI detectors are identical, we can multiply the solid angle with the number of detectors to get the eciency of the combined NaI detectors. 9

12 Figure 12: Singles and coincidence measurements for a 60 Co source using the Ge detector Rearranging the equation above we get: ω(e γ2 ) = N Ge C (E γ 1 ) N Ge S (E γ 1 ) Which we can use to nd the total eectiveness of the combined NaI detectors: Ω NaI (E γ ) = 26 6 ω(e γ) From the spectra in gure 12 we nd: NS Ge (1.17MeV ) = NC Ge (1.17MeV ) = NS Ge (1.33MeV ) = NC Ge (1.33MeV ) = And we get the solid angle of a typical NaI detector and the total eciency for the combined NaI detectors respectively: ω(1.17m ev ) = ω(1.33m ev ) = Ω(1.17M ev ) = Ω(1.33M ev ) =

13 4.2 TDC Calibration A TDC (Time to Digital Converter) spectrum shows the number of coincidences as a function of time delay. Figure 13 shows a TDC spectrum for a 22 Na source, with the Ge detector as start. We add a 16ns delay to the Ge detector, shortening the timespan between start and stop and expecting the TDC graph to shift to the left, as shows in gure 14. The amount of channels the peak has shifted from gure 13 to gure 14 is 33. With a 16ns delay that gives us a factor of a 1 = 0.48ns/ch. We want the spectrum to show the peak of the graph as time zero, meaning channel (centroid of unshifted spectrum) corresponds to time zero, and we have a calibration constant a 0 = a = 166. The FWHM of this spectrum is then found to be 35ns, this is the resolving time of our system. Figure 13: TDC spectrum for 22 Na with Ge detector as start Figure 14: TDC spectrum for 22 Na with 16ns delay on Ge detector 4.3 NaI Eciency at 511keV and 1275keV Using the CACTUS to measure a 22 Na source gives us singles and coincidence spectra for the Ge detector seen in gure Na decays in 90.5% of the cases with β + emission; this positron is annihilated in the source resulting in the emission of two 511keV photons with an angle of 180 between them. In the remaining 9.5% events electron capture occurs, emitting a photon at 1275keV. From section 4.1 we recall how we found the solid angle of a typical NaI detector, and total eciency of our combined NaI detectors. Using the same method now we nd: NS Ge (511keV ) = NC Ge (511keV ) = 497 NS Ge (1275keV ) = 5528 NC Ge (1275keV ) =

14 Figure 15: Singles and coincidence spectra for 22 Na with Ge detector. Which gives: ω(511kev ) = ω(1275kev ) = Ω(511keV ) = Ω(1275keV ) = The reason for the larger eciency at 511keV might stem from the fact that when there's a β + decay it always results in two 511keV photons, so even if one of the photons escapes detection there's still a chance the other one will register. We notice as well that the results we got from measuring 60 Co correspond to what looks like a decrease in detector eciency for higher energies. 4.4 Multiplicity We'll be looking at something called n-fold Ge spectra. n-fold means that n NaI detectors have to be in coincidence with the Ge detector. Specically we'll be looking at 0, 1 and 2 fold spectra; i.e. 0, 1 and 2 NaI detectors in coincidence with Ge detector, for 137 Cs, 60 Co and 22 Na, with multiplicities of M NaI γ = 0, = 1 and Mγ NaI = 2 respectively. Figure 16 shows us the n-fold spectra for 22 Na. In the lower left panel (0-fold) we recognize the 511keV, 1275keV and the 1460keV line from 40 K. The potassium line is expected to disappear for 1-, and M NaI γ 2-fold. Examining the 1-fold spectrum, however, shows us a count of about keV photons. These are most likely due to random coincidence. And 12

15 Figure 16: Lower left, lower right and upper left contain 0-, 1- and 2-fold spectra for 22 Na respectively. indeed, the 1460keV line has a count 20 in the 2-fold spectrum; that there's any count at all is probably because of chance interactions with background radiation or compton scattering. The results for 60 Co were as expected, with negligible counts for 2-fold. However, the detectors registered a count of 456 for 1-fold for 137 Cs, where we should be expecting none; we would expect the counts to be proportional with the relation f i = Ω i for i Mγ NaI, and f i = 0 for i Mγ NaI, where f i is fold i. Though the ratio of counts for the 662keV line in 0-fold (212882) and 1-fold (456) makes it likely that the unexpected counts are just a consequence of random coincidence. 5 Summary Through the course of this laboratory excercise in nuclear physics we've learned about detectors - particularly NaI scintillators and Ge semiconduction detectors; their advantages and disadvantages, and the appropriate use of the detectors; 13

16 when one needs higher resolution or better eciency, as well as the more technical aspect of using the detectors; calibration f.ex., and nding and calculating eciency and resolution, among other things. We've seen the eects of photoelectric eect, pair production and compton spread, as well as studied methods to reduce unwanted interference from these incidents. We learned the basics of coincidence techniques and n-fold measurements. Their applications for determining decay events in sources, as well as their use in nding eciency of single or multiple detectors. We also learned about the gamma multiplicity of isotopes. The excercise has also taught us about general lab work; keeping logs, writing reports and doing presentations, and proper conduct within a laboratory. 6 Appendix 6.1 Partial Decay Schemes 14

17 References [1] Magne Guttormsen, Hilde Nyhus - Guidance, counselling and advice [2] M. Guttormsen, T.S. Tveter, L. Bergholt, F. Ingebretsen, J. Rekstad - The Unfolding of Continuum γ-ray Spectra [3] W. R. Leo - Techniques for Nuclear and Particle Physics Experiments [4] K. S. Krane - Introductory Nuclear Physics [5] Oslo Cyclotron Laboratory webpage - [6] Wikipedia.org - Compton scattering Electron-positron annihilation Gamma spectroscopy Pair production Photoelectric eect Scintillator Semiconductor detector 15