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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
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The experiment is performed in the following steps:
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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 Co60 and Cs137 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.
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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 Æ 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.
Î 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.
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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.
Input scope
50 Ω
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).
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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.
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You can later read in the stored histograms again and edit or manipulate
them Æ click on Datei Æ 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 (Æglätten). This will allow to fit lines to the
histogram much easier.
Use the option ZOOM : VIEWÆZOOM 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.
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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.
Æ make a scetch in the logbook.
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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: Sr90 with an endpoint of
0.546 MeV and the spectrum of the daughter nucleus Y90 with an endpoint of
2.284 MeV. Both are 'allowed' Beta decays e.g. the form of the electron
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spectrum is given by dN(E)/dE ~ (E+mec4)* SQRT(E2 +2mec2)* (Emax-E)2
where E is the measured kinetic energy of the Elektron and Emax 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+mec2)*sqrt(E2+2mec2)] ) ~ (Emax-E) . If you plot K(E) it should be a linear
function of the measured energy E with intercept Emax 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.
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