Fast first practical help -- detailed instructions will follow- preliminary Experiment F80

Transcription

1 Fast first practical help -- detailed instructions will follow- 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: Two small plastic scintillation detectors are assembled (scintillator plate, light guide and Photomultiplier) using light tight tape by the students. Inspection of output pulses with a scope and measurement of average pulse height vs. high voltage Measurement of gamma rays from Co 60 and Cs 137 sources with an analog to digital converter, registration of the observed pulseheight spectra and calibration of the energy response of the 2 scintillators. Measurement of the high energy part of the electron spectrum of a Sr-Y beta source and determination of the endpoint. Measurement of the time resolution of a scintillation counter using two photomultipliers and two different discrimination methods Evaluation of the spatial resolution of particle transition from time measurements. Please prepare yourself before you start the experiment by reading the attached documentation and the printed document available I the FP, 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 log book. 1

2 Practical hints for the experiment: 1. wrapping of scintillators and connection of PM s Never touch the scintillator with bare hands use cotton tissues or gloves. Fat finger prints will lead to cracks in the surface of the scintillator and 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 scintillator or light guide again this would destroy total reflection. There should be no or minimal air gap between scintillator edge and light guide and also not between lightguide and PM cathode. A large gap would lead to light losses. If the gap changes during measurement then the counter efficiency will change the calibration is not stable. 2. oscilloscop operation and use The oscilloscope is an extremely valuable instrument. Never use a signal before you have seen it on the scope!! You must be sure that the signal has the right shape and amplitude and rate. use 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: Selection of display and trigger 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. 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 How to display two signals simultaneously 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. 2

3 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. 50 Ω Input scope 3. measurement of pulse height vs. high voltage Look at the pulses on the scope. Make a sketch of the pulse shape into the Logbook. Note down pulse length, typical pulse height and rise time of the pulses. Is the shape independent of the pulseheight? To make the measurement it is advantageous to use the pulse averaging option of the scope. Once you have chosen your working voltage fix the potentiometer and never change it again during all measurements. Else the calibration will not remain constant. 4.calibration of the energy measured in the scintillator 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?) Switch on the crate! 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 in the logbook. 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). 3

4 Data taking program and multi channel analyser Make yourself familiar with the data taking program and its important options: In the tool bar on top: VKA (multi channel analyser) look at measurement settings: you can choose the measurement time, 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 selected number of second or you can stop it by hand. Whenever you have to subtract background from a 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. You can later read in the stored histograms again and edit or manipulate them click on Datei open data source. You see 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). This will allow to fit lines to the histogram much easier. 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. Print out: If you are content with the histogram display on the screen choose the best vertical display by the scrollbar at the side of the window then copy the histogram to the Zwischenablage (intermediate copy). Open the drawing program or Word and paste the histogram to a new window. Add a title to the histogram and print it out. The printout will appear on the network printer on the same floor. 4

5 N ote: once you have an energy calibration from the first measurements ( and if you are sufficiently clever) then you can insert it into the program and measure further histograms using energy in kev directly on the horizontal axis. 5. measurement of time resolution: 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. 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. make a scetch in the logbook. 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. ==> please make the measurements using both discrimination methods and compare. Start the measurement by looking at the input and output pulses simultaneously on the scope. The scintillator pulse can be split at the scope using a T-connector such that part of the pulse can also be guided to the discriminator. Note: To do this you have to use AC-coupling on the scope. Look at the time jitter of the pulses which are not used for triggering. Do you see a difference for the two disciminators make sketches into the logbook. 6. data analysis: endpoint of Sr-Y spectrum The 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. Both are 'allowed' Beta decays e.g. the form of the electron 5

6 spectrum is given by dn(e)/de ~ (E+m e c 4 )* SQRT(E 2 +2m e c 2 )* (Emax-E) 2 where E is the measured kinetic energy of the Elektron and E max the maximal kinetic energy of the spectrum. (see any textbook of Nuclear Physics). In order to determine the endpoint it is best to generate a linear function of the energy. This is done in the socalled Kurie-Plot: K(E)=Sqrt([ dn(e)/de ]/ [(E+m e c 2 )*sqrt(e 2 +2m e c 2 )] ) ~ (Emax-E). If you plot K(E) it should be a linear function of the measured energy E with intercept E max in the energy range above the Sr endpoint. So fit a straight line to K(E) (after subtracting the background of course) to determine the endpoint. 6