--- preliminary Experiment F80

Transcription

1 --- preliminary Experiment F80 Measurement Methods of Nuclear and Particle Physics Introduction: This experiment is going to introduce you to important counting and measuring techniques of nuclear and particle physics. The basic detector used in this experiment is a plastic scintillator read out by a photomultiplier. This type of detector is rather simple, easy to build in very different sizes and geometries and is widely used especially to derive fast timing information if a particle hits the scintillator. Time resolutions of 100 ps can be reached if special care is taken. In this experiment scintillation counters are used to measure the energy of gamma rays and decay electrons. In a second step also the time resolution of scintillation counters is measured. The experiment is performed in the following steps: One plastic scintillation detectors is assembled (scintillator plate, light guide and Photomultiplier) using light tight tape by the students. Output pulses are inspected with an oscilloscope and average pulse height vs. high voltage of the pulses is measured Gamma ray signals in the scintillators from Co 60 and Cs 137 sources are measured with an analog to digital converter, the observed pulseheight spectra are recordes and used to calibrate the energy response of the 2 scintillators. The high energy part of the electron spectrum of a Sr-Y beta source is measured and it s endpoint determined. The time resolution of a scintillation counter is measured using two photomultipliers and two different discrimination methods The spatial resolution of particle transition from time measurements is evaluated. Please prepare yourself before you start the experiment by reading the attached documentation and the printed document available in the FP ( Radiation detection and Monitoring ), looking through the attached list of questions (which you must be able to answer). This may require to also have a look at the literature given at the end. Books can be found in the library of the FP. All measurements have to be documented in the log book. Also the spectra which you obtain should be printed out on the network printer and glued to into the logbook. Finally all results and evaluations have to be in this logbook.inbook 1

2 Practical instructions to perform the experiment: 1. wrapping of scintillators and connection to PM s Three scintillators are used: Thick scintillator: 9 mm thick Thin scintillator: 5 mm thick Scintillator for time measurements 20 cm long, 8 mm thick. Never touch the scintillator (and also not the light guide ) with bare hands use cotton tissues or gloves. Fat finger prints will lead to cracks in the surface of the scintillator. This destroys the total reflection of light. Have a look at the scintillator pieces you will probably see these effects due to mistreatment by your predecessors. Take care that the black tape will not stick to the scintillator or the light guide again this would destroy total reflection. There should be no air gap between the scintillator and the light guide and also not between the light guide and the PM cathode. A gap would lead to light losses. If the gap changes during measurement then the counter efficiency will change which wold lead to an unstable calibration.. Scintilator Lightguide Photomultiplier Socket HV divider Teflon tape Never take apart! Black tape 1st layer Tape all pieces together Figure 1 Black tape second layer signal HV 2

3 You are expected to wrap the thick scintillator detector yourself as shown in the figure above. This gives you a chance to see all pieces of the detector and also to learn something about light guiding. In addition some practical work should be fun to you. The other detectors are expected to be available fully assembled. All three pieces scintillator, light guide and PM - have to be wrapped first by the white teflon tissue tape. Each winding should overlap in area with the old one by about 1/3. What is the purpose of this tape? Next you have to wrap the pieces with the black scotch tape twice to make the counter light tight. The second layer is wound with opposite winding angle with respect to the first layer as shown on the drawing. Finally connect the three pieces with black scotch tape. Make sure that there is good mechanical contact at all surfaces to avoid air gaps which lead to a loss of light. At the end the counter should be mechanically stable and absolutely light tight, else you will not be able to measure small count rates. 2. First look at output the PM pulses vs. high voltage Set the potentiometers (voltage) of the HV power supply to zero. Put the thick counter to the black box (presently a cartoon box) and use the Co 60 source to illuminate it. Then connect the HV cable of the thick counter to one output of the HV power supply. Connect the output of the PM to the scope, switch on the HV and raise it. Starting at around -600 V you should start to see pulses. This is the moment when you should make yourself familiar with the scope and its various options which you will need: Oscilloscope operation and use The oscilloscope is an extremely valuable instrument. Never use a signal before you have seen it on the oscilloscope! You must be sure that the signal has the right shape, amplitude and rate. You better spend some time to make yourself knowlegable how to use the scope, this is not trivial the scope has a lot of knobs and options. You must be familiar with the following operation steps so try out all these operations: Selection of display channel Selection of time axis (horizontal sweep)- always look to the written screen information. Example: H: 20 ns. This means 1 cm corresponds to 20 ns. 3

4 Selection of vertical axis (voltage) for the selected channel. Screen display : example CH1 20 mv. This means 1 cm corresponds to 20 mv. Always use the full scale to view the pulses to have good resolution. Selection of trigger source (CH1 or CH2) and adjustment of trigger Threshold Switch on the persistance option which allows you to see many pulses simultaneously on the scope. The persistance time can be regulated with the turning knob at the top of the front panel. Learn how to go to single pulses again. How to display two signals simultaneously? You can independtly choose vertical and horizontal axes for the two input channels select the channel first before you move the knobs! The scope has additional useful options: e.g. determination of average pulse height over 256 pulses this is very useful for the first measurement: pulse height vs. HV. The input cables of the oscilloscope have to be terminated by 50 Ohm ( the impedance of the coaxial cable)! This is done by using a T- switch with a 50 Ohm resistor on one end and the cable at the other. This serves two purposes: a) the resistor acts as working resistor which provides the input voltage. b) termination with 50 Ohms avoids reflections of the pulses in the cable. (for experts: the basis of the PM has no working resistor see the corresponding circuit diagam, this resistor has therefore to be provided independently. In principle you could use the input resistor of the scope but this works only with AC coupling). PM Input scope 50 Ω The output pulses are expected with a typical height of ~20 mv and a pulse length of ~20 ns. Measure the average pulse height vs. HV in the range between -900 V and V and record it in form of a table in the logbook. Why does it depend so strongly on the HV? Make a graph of the result into the logbook by hand. Choose a working voltage In this range and keep it for the rest of the measurements! (block the potentiometer). For this measurement you can profit very much by using the averaging mode of the scope.note: the average pulse height depends on the threshold! Make a sketch of the pulse into the logbook with time and pulseheight in- 4

5 formation. You will have to find this kind of pulses again and again in the course of the 4 half days. Take away the source (screw it back to its absorber block ) and measure the dark pulse rate of the scintillator and their average pulse height. This rate should be low, else you have a light leak! Repeat all steps for the thin scintillator. At the end of the first half day these measurements and the graphs have to be in both logbooks! 3. calibration of the energy measured in the scintillator with Co 60 and Cs 137 sources Co 60 and Cs 137 are two commonly used γ refence sources. You find the decay schemes in the attached document Radiation Detection and Measurement by G.F. Knoll of which you can get a paper copy in the FP. Co 60 has two γ lines at and MeV Cs 137 has one γ line at 0.66 MeV This energy range is the most difficult one for spectroscopy since the dominant interaction process of γ s in this energy range is Compton scattering which leads to a continuum of electron enegies and therefore to a continuous pulse height spectrum up to the compton edge (see section theoretical background and appendix1. The energy calibration is therefore based on the evaluation of the measured pulse height spectrum and the determination of the Compton edge. You are expected to calibrate both the thick and the thin scintillator detector. The experimental steup is shown in figure 2. Put the thick scintillator and the Co60 source into the box. Switch on the crate! Connect the PM output cable to the input of the Amplifier in the NIM crate. Choose the right polarity of the input pulse! (is it positive or negative?). Choose an amplification of 100 (turning knob on top of module). Look at the output pulses on the scope using the Uni(polar) Output. Pulse length and pulse form is now very different! Find the best scope settings and make a sketch of the pulses in the logbook indicating also pulse height and pulse length. If you are satisfied with the pulses and rates then connect the output cable of the amplifier to the ADC input at the back of the PC ( the input card is labelled with ADC). This is the time to make yourself familiar with the data taking and spectroscopy program. 5

6 HV1 HV2 approx V Co and Cs sources for calibration of the energy scale measured by ADC Sr Y source for measurement of energy deposition Scintilator Sr Y source PMT2 ADC Amplifier shaper PC with spectroscopic program A=100 shaping 1micro s Peak amplitude proportional to the integral of original pulse Figure 2 Data taking program and multi channel analyser Make yourself familiar with the data taking program and its important options: The toolbar on top allows you to start the most important options. Click fields are given below in red color. Datei Datenquelle öffnen. In the window choose either detector (click on PHA) to start reading in detector data ( or choose CAM to read in a data file from disk ). Click on VKA (multi channel analyser). Look at measurement settings: you can predefine the measurement time (what is the difference between live and total?), the number of channels in the display... In the tool bar VIEW you can choose linear or log vertical scale, in the submenue View->OPTIONS you can choose different ways to show the histogram. Recommended: draw line but try it out. Start the measurement- you will see a live update of the histogram The measurement will stop after the preselected number of second or you can stop it by hand. Whenever you have to subtract background from a 6

7 spectrum you must make sure that data is recorded over exactly the same time period. If you have recorded a useful spectrum which you want to analyse later then always store it in a folder with a meaningful name on the PC. Data files are recorded in the folder F80. Create a new folder with your name and store your measured files in this folder. (Datei speichern) Please create a new folder (best with your name) on the PC: Got to folder F80, click on Datei Neu and insert your name. In furture store your files in this ew folder. You can later read in the stored histograms again and edit or manipulate them click on Datei ( Öffnen einer Datenquelle ( click on CAM Open you file and read in your spectrum again. Now you can subtract two spectra from each other if needed ( click on Analysis). You can also edit your spectrum. It is recommended to go to Options and use the possibility to smooth the histogram ( glätten) which essentially means averageing several neighboring bins. This will allow to fit lines to the histogram much easier later. (Warning: it may happen that your spectrum gets distroyed in this step, just read it in again and try with new settings). Use the option ZOOM : VIEW ZOOM and select the part of the spectrum which you want to use. Especially useful if you have to determine the end point of a spectrum. But print out always both the full spectrum and the zoomed part. You will see both the original and the zoomed spectrum (this works also during data recording). Print out the spectrum using normally a log scale : If a measurement is finished or if you have read back a stored measured histogram and if you are content with the histogram display on the screen choose the best vertical display window by the scrollbar at the side of the window then copy the histogram to the Zwischenablage (intermediate copy) by ALT COPY on the keyboard. Open the open source drawing program or Word and paste the histogram to a new window. Add a title to the histogram (this is the only safe way not to mix up printed histograms) and print it out. The printout will appear on the network printer on the same floor. 3.1 Start the Co 60 measurement with you preferred VKA settings for a time period of about 1000 seconds. Choose the best plot display, store the histogram on the PC and print it out two times 1 copy for each student. 3.2 repeat all steps for the Cs 137 source. 3.3 Repeat all steps for the thin scintillator detector using both sources. 7

8 Analysis of the printed plots: this has to happen before you go to the next measurement: Energy spectrum of beta Sr-Y source, latest end of the second half day. All results have to appear in both logbooks, the plots have to be glued into the log book. The analysis of the plots has to be done independently by each student! Have a look at the measured logarithmic pulse height spectra which shows the number of entries vs. channel number. What is the interpretation of these spectra? Have a look again at appendix 1 where the ideal energy spectra are schematically shown. Determine the Compton edge usig.the printed log plot in the following way: Roughly the spectrum near the end can be described by two straight lines, one describing the flat top and one the falling edge of the spectrum. So draw two lines to the spectrum and determine at which channel number they intersect. You can approximately identify the intersection point with the Compton edge (why?). Calculate the electron energy at the Compton edge using the known gamma energy and the Compton formula (theory section). This gives you a calibration point energy/channelnumber. Try to estimate the error on this calibration point. (we don t expect a fit to the data, though you of course can do that if you export the spectrum to your CIP pool account via network- ask the assistant how to do that). For both scintillators separately: Draw by hand the calibration points ( energy vs. channel number) into a plot in the log book. Determine a straight line through these points using also the origin as a constraint - we expect channel number zero for zero energy. Note the energy calibration constants for both detectors in a box. This is the final result of this part of the measurement. You are encouraged to add a few remarks. For ambitious experts: once you have an energy calibration from these measurements ( and if you are sufficiently clever) then you can insert it into the data taking program and measure further histograms using energy in kev directly on the horizontal axis. 4. Measurement of the beta spectrum of a Sr-Y source. The decay electron spectrum is composed of 2 beta spectra: Sr 90 with an endpoint of MeV and the spectrum of the daughter nucleus Y 90 with an endpoint of MeV. 8

9 Both are 'allowed' Beta decays e.g. the form of the electron spectrum is given by dn(e)/de ~ (E+m e c 4 )* SQRT(E 2 +2m e Ec 2 )* (Emax-E) 2 where E is the measured kinetic energy of the electron and E max the maximal kinetic energy of the spectrum. (see any textbook of Nuclear Physics). A measured spectrum is shown in figure 3. Figure 3 You are now expected to measure this spectrum and to determine the end point energy with the two scintillators using their energy calibration. Principle of detection: the decay electrons traverse the scintillators and loose energy by ionisation. The light output of the scintillator is proportional to the ionisation. If the electrons are therefore stopped in the scintillators you will be able to measure their kinetic energy. Check therefore the range of the decay electrons in the scintillator material using the range curve of figure 4. What is the maximum range for electrons near the end point? What do you expect for the pulseheight in the thick and thin scintillators? 9

10 100 Range of atomic particles in Premium plastic scintillator 10 e Range (mm) µ d α Energy (MeV) figure put the thick scintillator detector next to the Sr-Y source and the thin detector behind it and switch their HV on. Look at the shaped amplifier output pulses simultaneously on the scope by triggering on the thick scintillator. Do you see simultaneous pulses in both scintillators? What do you expect? Note down your observations! Measure the spectrum of the thick scintillator for a fixed time of ~1200 s and store it. Take off the Sr source and repeat the measurement under identical conditions. This determines the background spectrum and store it. Repeat both steps for the thin scintillator. Note:the measurement of the background spectrum has to be done under identical conditions and very near in time to exclude pulseheight trips e.g. due to temperature changes. 4.2: evaluate the end point of Sr-Y in an approximate way before you start the next measurement: 10

11 In a first step you have to subtract the background spectrum from the spectrum with source for both scintillators. This is done by loading the source spectrum and displaying it in the best way. Recommended: display also a ZOOMed spectrum in the region near to the end point. Then go to Analysis subtract. What is the origin of the background? Why does it disturb this measurement? Please print out both the original spectrum and the subtracted spectrum Try to extrapolate the subtracted energy spectrum to the endpoint. This is best done in a ZOOMed spectrum near the endpoint in a log plot. What form of the spectrum do you expect? Determine the end point energy graphically and estimate its uncertainty. Compare to the expected value. NOTE: If the spectrum of the second scintillator after subtraction shows significant energy then you have to add this energy to determine the end point energy! 4.3: repeat all steps by interchanging the thin and the thick scintillator. What do you now expect for the relative pulse heights of the two scintillators? Now you will have to add the end point energies of the two spectra to get an energy measurement of the end point. The measurement and its analysis have to be finished before you start the last part measurement of time resolution. This is before the last half day! 5. measurement of the time resolution of a scintillation detector A longer scintillator (20 cm) is available which is read out by two photomultipliers, one on each end. Discriminators are used to derive a norm pulse (discriminator output) whose leading edge gives the time stamp for the arrival of the scintillator pulse. To measure the time difference between the two norm pulses we use a Time to Amplitude Converter (TAC). This is the 3 unit wide insert in the crate. One signal is used to start the time measurement, the other is used to stop it. (Use the corresponding inputs for these signals. BUT: The stop signal has to come after the start signal under all circumstances! This is done by delaying the stop signal using the delay boxes in the crate. Use the Sr-Y source for this measurement. 11

12 You are expected to use two type of discriminators to derive the norm pulses for the time measurement. They use different ways to determine the arrival time of a pulse. 5.1 Time resolution using a leading edge discriminator The experimental setup is shown in figure 5. In a leading edge discrimination, the time pulse is generated when the input pulse exceeds a fixed threshold. Pulses of different height lead to time stamps which depend on the pulse height - high pulses cross the threshold earlier than low pulses. HV1 HV2 PMT1 Scintilator Sr Y source PMT2 min. 50 mv min 50 mv Discriminator Constant fraction or Leading edge Adjustable delay approx. 16 ns Discriminator Constant fraction or Leading edge start stop Time Amplitude Convertor (TAC) out Use 2 delays to calibrate the time measurement using the ADC ADC PC with spectroscopic program Figure Put the source to the middle of the scintillator the two PM s should see the light pulses simultaneously. Check this by looking at both output pulses simultaneously on the scope. Connect the two scintillator 12

13 signals to the inputs channels of the leading edge discriminator module. Connect the OUT channels to the two scope inputs. Trigger on channel 1 and look at both channels simultaneously using persistence display. The pulses of channel two are fluctuating in time they show a time jitter. Read off the width of the jitter- it is a measure of the time resolution. make a sketch in the logbook including time indication connect the discriminator output of one channel to the START input of the TAC module. (see figure above). Connect the discriminator of the second pulse to the input of the delay box 1. Connect the output of delay box 1 with the input of delay box two. Connect the output of delay box 2 to the stop input of the DAQ module. Set all delay buttons to OFF (no delay). Look at the TAC output pulse on the scope. Add delays by switching the buttons in the delay boxes until you see a sizeable pulse. Connect the TAC output to the ADC input at the PC and start a measurement. Choose full measurement range in the VKA menu. Add or subtract delays until the measured pulses appear in the middle of the measurement range. This is now defined as zero time difference. 5.2 Measure the time resolution of the setup. Take a spectrum for at least 600 s. The width of this spectrum measures the time resolution! Store the spectrum (uses ZOOM and linear scale). Print it out twice, one copy for each student. 5.3 Calibrate the time ( time difference vs. channel number) This is easily done by taking off or adding known delays to the stop pulse in the delay boxes. Add 4 ns delay. Do not clear the old measurement (do not click on löschen but click on start. You will see a new peak coming up at a higher channel number!. The difference in channel number of the two peak positions corresponds to a time difference of 4 ns!. Repeat the same for -4 ns and choose other delays which look reasonable to you. You must have a calibration point near the observed time resolution! Print out the spectrum (after writing the time delays into the graph!) and determine the peak positions of all peaks. Use the original setting as corresponding to time difference zero. Make a table: delay in ns vs. the channel number and plot the calibration curve into the logbook by hand. Evaluate the calibration constant from the slope of a straight line. Determine the time resolution (in ns) by measuring the Full Width Half Maximum ( FWHM)of the measured time spectrum. How is this measure related to the standard deviation of a Gaussian distribution? 13

14 5.4 Evaluation of signal velocity in a scintillation detector The light pulses generated by an electron in the scintillator propagate through the scintillator if they stay within the total reflection angle. They have of course a finite velocity. Try to estimate the travelling time over a length of 20 cm. Is this difference measureable with the present setup? Put the source near the end of the scintillator (not the light guide!). measure its position relative to the middle and record it in the logbook. Take a spectrum over 600 s. Evaluate the time difference relative to the middle position. Repeat the same step by putting the source to the other end of the scintillator. Determine the velocity of light in the scintillator and explain the result. 5.5 time resolution using a constant fraction discriminator. Repeat all steps 5.1- to 5.4 by using the constant fraction discriminator. Is it necessary to repeat the time calibration? Constant fraction discrimination: the time stamp is set when a constant fraction of the integrated pulse (const. charge fraction) has been detected. This pulse stamp is much less dependent on the pulseheight and should therefore give much better time resolution Compare the two ways of discrimination! The difference should already be visible on the scope. Summarise the results of the time measurements in a short table in the logbook and discuss the results including propagation speed. 5.7 Spatial resolution from a time of flight measurement. Very often the fast scintillation detectors are used to measure time of flight for a particle which together with a momentum measurement allows then to determine the mass of the particle. What is the arrival time difference of a pion and a kaon of identical momentum of p=1 GeV/c if their flight path has a length of 1.5 m? How well can you separate the pion from the kaon if you insert the time resolution of your constant fraction measurement? 14

15 Calculate the difference and evaluate the separation in the log book. The masses are : m π = MeV, m K = (hint: use β=v/c=p/e, where E is the total energy of the particle ). In long scintillators which are read out on both sides the position of the particle traversal can be determined by measuring the time difference of the arrival times in the two PM s. Use the measured time differences for the three source positions of your measurements with the constant fraction discriminators to determine the spatial resolution of such a measurement e.g. how well could you determine the position of the source if you measure the time difference in the two PM s. These evaluations should be finished before the end of the last half day. So you can get the final Testat on the fourth half day. Wish you fun and success during the experiment Good luck 15

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