FYSP106/K3 GEIGER & MÜLLER TUBE. 1 Introduction. 2 The equipment

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1 FYSP106/K3 GEIGER & MÜLLER TUE 1 Introduction In this measurement you get familiar with Geiger-Müller tube. The dead time, the range of beta-radiation in medium and the activity of the radiation source are measured. Literature related: - K. Krane, Introductory Nuclear Physics, Chapter 7.2 Gas-filled counters 2 The equipment The Geiger-Müller detector (Geiger counter) is a simple device without complex electronics. It is mostly used in activity measurements and meters used in controlling the radiation. From each detected event the tube supply the same pulse to the counter thus the only supplied information is the number of events. The energy of the radiation can t be measured. The tube itself is filled with noble gas (neon) and has the shape of a cylinder, a thin anode string in the middle. The cathode is on the outer shell (stainless steel). On the end of the cylinder there is a radiation permeable window (in this case beryllium, surface 2 density 2 mg cm ). The radiation (, or ) hitting the tube ionises (neon-) atoms. ecause of an electric field, the electrons are thrown towards the anode and ions the cathode. These charges ionise more atoms on their way and so an avalanche of ions and thus measurable current is born. Ions are neutralised when they get a missing electron back but remain in excited state. To absorbe this energy and to stop the avalanches there is also some halogen gas in the tube as a quenching gas. During the avalanches the electric field is weakened and the tube is incapable to measure new events. This time is called the dead time. In this Laboratory work a Geiger-Müller tube and counter / HV source made by Spectech is used.

2 The setup is shown in Fig. 1. ccording to the manufacturer, dead time of this setup is 200 μs. Figure 1: Photograph of the equipment. 3 The dead time When an impulse hits the detector, it is for a while incapable to measure new impulses. This time is called the dead time. ecause of this dead time, the measured number of pulses N is smaller than the real number N 0. The connection between these two can be written as N N 0 1 N, (1) t where t is the total time of the measurement. The implication of the dead time is emphasized when the number of events is big. This is the reason why the distance between the source and the detector must be big enough. 3.1 Measuring the dead time 204 In measuring the dead time, two pieces of crescent shaped Tl (thallium) sources are used. First the numbers of pulses are measured from both pieces separately N and N.

3 The the measurement is repeated, sources side by side N, when the result is smaller than the sum of previous measurements N N. This difference tells something about the magnitude of the dead time. The measurements of both pieces ( and ) obey the equation (1) N N t N t 0 1 N and N0 1 N, (2) and so does the simultaneous measurement of pieces N 0. (3) t N N 0 1 N y solving from these equations (homework) we obtain N N N N t N N N N (4) N N N 4 eta radiation There are two ways of beta emission: M D Z N Z 1 N1 M D Z N Z 1 N1 e e The energy released from emission is converted to the kinetic energy of the -particle a.k.a an electron, e (positron, e ) and the antineutrino, (neutrino, ). This is the reason why there are no sharp peaks as in gamma spectroscopy, but a continuous

4 distribution of particles with different energies. n absorbtion law doesn t hold for beta radiation and it is not relevant to talk about the absorption coefficients or half-value layers. Instead, natural for beta emission are the maximum energy of -particle and the range in a certain medium. The range is the thickness of a medium, which stop s all of the -particles. The absobtion of -particles happens in collisions with the atoms of the medium. ecause electrons are very lightweighted, they change their direction violently in each collision. So the real length of the electrons trajectory is much longer than the range. 4.1 Determining the range of the beta radiation and the maximum energy In the measurements a Sr strontium Y yttrium -source is used, which decays as in Fig. 2. The way of decay is written on each transition, (maximum) energy and the relative probability of the decaying brach. Notice, that the source emits almost only - particles. Figure 2: The decay scheme of the beta-source.

5 In the measurements, aluminum discs (with known surface density) are placed between the source and the detector. The radiation, which goes through the discs is measured. y plotting the results in half-logarithm co-ordinates, you get a graph as in Fig. 3. The measured number of the pulses decreases until it reaches the level of gamma radiation which is caused by the background radiation and the radiation from the source. The slight decrease in gamma level is caused by the absorption of the gamma radiation in aluminium. The electrons emitted from the source are also absorbed in the l casing of the source (surface density 130 mg 2 cm ). When determining the maximum energy, the surface density of the shell must be added to the surface densities of the absorbers. In addition, -particles are absorbed into the air and the window of the Geiger-Müller tube. This, however, is insignificant and can be ignored. Figure 3: The absorbtion curve of the beta radiation In order to determine the range, the extrapolations are drawn to the descending and horizontal parts of the absorbtion curve, whose intersectional point is the practical range R. The maximum range R k max, from which the maximum energy of the -particle can

6 be calculated, cannot be determined as precisely. The determining is done by finding out the position, where the -particles can no longer be distinguished from background (point b in fig. 3). The relation between the range and the maximum energy of the -particles is shown with moderate accuracy by the equation Rmax Emax, (5) where energy is in the unit of MeV and R max of mg/cm 2. Constants and are experimental parameters which are typical for a certain medium. For aluminium =540 and = Determining the activity of the beta source The activity of the source tells how many events of decay occur in a unit of time. The most common unit is ecquerel q (1/s). The activity is determined from the absorption curve above. First, the number of pulses corresponding to the zero-absorption thickness is read from the curve. Perform the following corrections: 1. Dead-time correction. The counter only counts part of the impulses hitting the detector, because part of the time it is not functioning. Corrected number of pulses can be obtained from eq Correction of the gamma background. The increase in the number of the pulses N G, can be obtained by extrapolating the background to zero-absorption width, as in fig Solid-angle correction. The source emits electrons in every direction at the same probability. That is why the total amount of emissions can be obtained by

7 multiplying the number of the pulses with the relation of the areas of the sphere 2 and the Geiger tube window. The area of the window is 4,9 cm. In addition, a safety net covers 23 % of the window, so the effective area of the window is 4,0 2 cm. ccording to the manufacturer s notification, the nominal activity of approximately 74 kq on Sr -source was Some technical details are given in Table instructions below.

8 Table instructions 1. Device: Check that G&M tube is connected with a NS cable to the counter. Turn the SPECTECH ST360 COUNTER on. Press DISPLY SELECT -button until it shows TIME. Set the PRESET TIME to 60 seconds. Next press DISPLY SELECT button until red led moves to HIGH VOLTGE. Increase the voltage with UP button to 900 V. Press DISPLY SELECT button until red light goes to COUNTS. Now the COUNT button starts a measurement lasting for 60 seconds. 2. Determination of dead time: Place the disc halves (green side towards the counter) always to the same position. djust the distance between the counter and source such that number of counts is about per minute with one radiation source. 3. Measurement of absorption curve of beta radiation Measure the surface thicknesses of aluminum absorbers [mg/cm 2 ]. Same value can be used for all absorbers as the diameter. Place the source above the GH tube so that there is enough space for all the absorbers. Don t move the source during the measurements. Measure the distance between the outer point of the source and the edge of the plastic box, and add 15 mm (additional distances in the source and counter) to that. The effective area of the window in the GM tube is 4,0 cm 2.