CHARACTERISTICS OF GEIGER – MULLER COUNTER Task: 1. Get familiarized with the function of GM counter. 2. Measure the characteristics of GM tube and, based on the measurement, determine the value of threshold and operating voltage, length and plateau sloping. 3. By the method of two specimens, determine GM counter resolution time τ. Introduction: Geiger-Muller gas detector is a cylindrical condenser in which the space between the electrodes is filled up with a suitable type of gas. When voltage is hooked up to the condenser, the size and distribution of the electrical field between electrodes will fundamentally influence the characteristics of the detector. The gases are under normal conditions isolators. When ionizing radiation impinges on the working area of GM tube hooked up to high voltage, a part of originally neutral atoms and molecules changes into positively and negatively charged ions and electrons through which the gas inside the tube becomes conductive. With a certain increase in the intensity of the electrical field, corona discharge occurs in the detector area. GM counters work in this regime. If a radiator is placed near GM counter, which is hooked up to low voltage (200 V), the counter will not register any impulses. By gradually increasing the voltage of the counter, we find, that at a certain value of the voltage the counter starts registering a certain low number of impulses. Thhis voltage value U0 is called the threshold value (see the Fig.1). Fig. 1: GM tube characteristic. If the GM tube voltage is increased, the initial fast growth of registered impulses passes into the area, where their number almost does not depend on the increasing voltage. This part of the characteristics is called plateau. Plateau lies between A and B poins on the characteristics. In regular types of GM tubes used in practice, the length of plateau ranges from 100 to 200 V. The operating voltage UP is selected somewhere in the middle of the plateau. With an increase of the voltage above US an independent discharge in the tube may occur, which could results in tube damage and should not be experienced under any conditions. Plateau sloping S is determined from the characteristics and is usually indicated in %. It may be calculated by the formula ∆N S = -------- . 100 [%] <N> where ∆ N is the increase of registered impulses with the voltage increase in GM tube by 100 V, < N > is the medium number of impulses registered in the plateau area. When n particles from the radiator impinge on GM counter during the detection interval, then the number of actually registered impulses will be smaller. It is because GM counter is not able to register any impulses during a certain time period. This time period is called counter dead time τD. The time interval that elapses between the moments when the counter is able to register two different particles is called the resolution time τ of GM counter. In quality GM counters, τ ≡ τD. τ is determined by the method of two specimens. The relationship between n – number of particles impinging on the GM tube and N – number of registered impulses is as follows: N n= 1 − N ⋅τ If, during the detection interval, n1 particles from the first preparation impinges on the GM tube (N1 impulses registered), n2 particles from the second preparation (N2 impulses registered) and if n0 impulses correspond to the background (N0 impulses registered), then n1 + n2 = n12 + n0 where n12 is the number of particles impinging on the GM tube from both specimens (when N12 is the number of impulses). By modification of both relationships we get: N1 + N2 – N12 – N0 τ = --------------------------- N12 (N1 + N2 - N0) Basic steps order for preparation of nuclear counter for measuring (see the detail instructions in the next paragraph): a) Set high (operating) voltage regulator at point 200 of the scale b) Switch apparatus on c) Make sure that high voltage switch is off (control light not on) d) Reset display to zero e) Determine time pre-selection (time of impulse registration) f) Set predetermined high (operating) voltage g) Press high (operating) voltage switch (red control light on) h) Press start button, counter starts counting impulses. Apparatus start-up detail instructions: 1) Make sure that high (operating) voltage regulator for GM tube (17) is set at point 200 of the scale. 2) Hook up apparatus to electrical network and switch it on by pressing button no. 11. Apparatus stays on all the time during practice. 3) Make sure that high voltage is switched off (high voltage supply to GM tube is switched on by pressing button no. 6). Red control light inside button must be off. Control lights no.5 indicate polarity of high voltage for GM tube. Left light must with indicated + polarity be on. 4) Press - button no.8 – resetting. Counting decades no.19 get set to zero. Zeros appear on display. - push in button no.9, after HV is switched on, pointer instrument no.18 will indicate high voltage value. - push in button no.10. Operation of instrument with time pre-selection is then set up. - remaining buttons, except no.11, are not pressed in. 5) Set time pre-selection switch no.13, 14 into prescribed position. Upper row of numbers applies (for example 6 x 10’s – it means that measuring time 60 sec is pre- selected). 6) Regulator no.12 – discrimination level – at 25 gradations = 25 %. During manipulation with regulator, it is necessary to release break (arrestment) of regulator – with black lever on right hand side. It must be shifted into upper position. Otherwise regulator will not turn, it can get damaged by forcing it. 7) Switches no. 15,16 – time constant and integrator range – may be in any position. Integrator is not in operation. 8) Switch no.2 – width of window of analyzer – into DIS position. Analyzer is this way out of operation. Its use is pointless because GM tube (unlike scintillation detector) is not proportional detector, it means that it registers particles with different energy values as impulses of the same size. With using GM tube as radiation detector it is not possible to differentiate particles according to energy and it is not possible to measure spectrum. 9) Amplification – switch no.3 into position 7,5 10) Switch HV on by pressing button no.6 (red button light is on) and set voltage for GM tube to prescribed value with regulator no.17. Actual voltage value in Volts is double value set on regulator in gradations. In upper part of scale there are hundreds, in bottom part there are tens and ones. While manipulating with regulator, it is necessary to release break (arrestment) of regulator. It is black lever on right hand side. It must be shifted into upper position, otherwise regulator will not turn, it can get damaged by forcing it. Procedure: Task – item no.2 - measuring GM counter characteristics 1) For measuring GM counter characteristics, radiator β (137 Cs) is selected 2) In accordance with above procedure, switch on apparatus and set HV to starting value 200 V and detection interval 60 sec. 3) Gradually increase voltage and register first registered impulses with reproducer switched on. Because individual impulses are registered at HV value 1,200 V, you can expect threshold value U0 from 1,300 V. Measure number of impulses 3 times in 60 sec detection interval. 4) Gradually increase GM tube voltage in 20 V (10 gradations) intervals and again measure 3 times registered impulses. Increase voltage up to value slightly higher than Us value (approx. 1,520 V). Maximum voltage value which must not be exceeded is 1,540 V ( exceeding this value, when corona discharge occurs in GM tube, leads to its permanent damage). 5) From average values of numbers of registered impulses, draw GM counter characteristics and evaluate Up operating voltage, (approx. at half of plateau), of plateau sloping. 6) With operating voltage, we evaluate GM resolution time τ. Task – item no.3 – Determining GM counter resolution time τ For measuring, use two radiators β – (1. – 90Sr, 2. – 137Cs) 1. Measure number of impulses corresponding to (N0) background, number of impulses when radioactive specimens are separately inserted (N1 and N2) and number of impulses corresponding to both inserted simultaneously (N12). Measure all values 3 times. Select 60 sec detection interval and express average values of number of impulses N in number of decays per sec. 2. Maintain following geometry of preparation insertions: - insert both simultaneously symmetrically toward GM counter tube (see Fig. 2) and measure N12 Fig. 2.: The GM tube and specimens position (top view) - subsequently, measure N1 and N2 and at the same time maintain original position of both radioactive specimens. 3. From measured average values, calculate GM counter resolution time.