GM Counter-Physics-Lab Report, Exercises of Physics

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To Study Characteristics Of GM Tube And To Find Half

Life Of A Radioactive Isotope

Submitted to:

Dr. Asloob Ahmad Mudassar

Submitted by:

Yasir Ali

M.Phil. Physics DPAM

PIEAS

Introduction:-. In this experiment we studied Geiger Muller tube as radiation detector. Geiger counter is a detector which can detect a radiation and measure it activity rate. In this experiment we used Geiger tube, finding its dead time and also its operating voltage. Similarly in this experiment we studied some radioactivity and with the help of GM tube half life of some radioactive isotope was measured. Radioactivity:-. In nature there are some elements which are unstable. Unstable mean they posses some extra amount of energy in the nucleus. This energy makes them unstable. These elements emit energy by some process. This phenomenon is called radioactivity.

Radioactivity was discovered by Becquerel in 1896 when he wrapped a photographic plate around uranium sample. The radiation emitted from uranium was later recognized as alpha particles. Later on, other type of radiations was also discovered known as beta particles and gamma particles.

Unstable elements get rid of extra energy by some processes and they decay into some other element. This process is called radioactive decay and elements are called radioactive elements. There are three basic processes by which unstable elements decay to some other elements which may be stable.

1. Alpha decay:-. Emission of alpha particles from a heavy unstable nucleus is called alpha decay. Alpha particles have two protons and two neutrons, so in this process atomic number of parent nucleus is changed by 2 and mass number is changed by 2. Some energy is also emitted in this process. Uranium, plutonium americium etc are alpha particle emitters.
2. Beta decay:-. Elements which emit beta particles from their nucleus are called beta emitters and process is called beta decay. In this process either a positive electron (positron) is emitted or a negative electron is emitted. In positron emission a proton is converted into neutron and positron is emitted. This process will convert atom to next below atomic number element. In electron emission process a neutron is converted into proton and an electron is produced. Ad electron has energy very less so it cannot stay inside nucleus so they are emitted and so atom is transformed into next above atomic number element. There is another process which takes place in beta decay that is called electron capture. In this process an electron from electronic shell is swallowed by nucleus and a proton is converted into neutron so atom is transmuted into next below atomic number element.
3. Gamma decay:-. Atoms emitting gamma radiation is called gamma emitters and phenomena is called gamma decay. During some nuclear reactions or in some decay processes discussed above, atom after reaction may have some excess amount which it can emit through nuclear transitions, called gamma radiations. In gamma decay no nuclear transmutation takes place but atoms are only transformed from excited to ground states.

Radioactivity decay law:-. By above decay processes atoms of one element are transmuted into elements of other elements. So we can say that atoms have some life. In radioactive we use a term called half life. Half life is time interval in which half number of atoms of one element is transformed to some other element. In next half life, half number of atoms of the

current is produced. The current is in pulse form and scaler counts the current pulses, and one obtains a "count" whenever radiation ionizes the gas. The apparatus consists of two parts, the tube and the counter + power supply. The Geiger-Mueller tube is usually cylindrical, with a wire down the center. The counter + power supply have voltage controls and timer options. When ionizing radiation such as an alpha, beta or gamma particle enters the tube, it can ionize some of the gas molecules in the tube. From these ionized atoms, an electron is knocked out of the atom, and the remaining atom is positively charged. The high voltage in the tube produces an electric field inside the tube. The electrons that were knocked out of the atom are attracted to the positive electrode, and the positively charged ions are attracted to the negative electrode. This produces a pulse of current in the wires connecting the electrodes, and this pulse is counted. After the pulse is counted, the charged ions become neutralized, and the Geiger counter is ready to record another pulse. In order for the Geiger counter tube to restore itself quickly to its original state after radiation has entered, a gas is added to the tube. For proper use of the Geiger counter, one must have the appropriate voltage across the electrodes. If the voltage is too low, the electric field in the tube is too weak to cause a current pulse. If the voltage is too high, the tube will undergo continuous discharge, and the tube can be damaged. Usually the manufacture recommends the correct voltage to use for the tube. Larger tubes require larger voltages to produce the necessary electric fields inside the tube. In our experiment we also determined the proper operating voltage. First we placed a radioactive isotope in front of the Geiger-Mueller tube. Then, we will slowly vary the voltage across the tube and measure the counting rate. Counting rate against the increasing operating voltage give optimized operating voltage. For low voltages, no counts are recorded. This is because the electric field is too weak for even one pulse to be recorded. As the voltage is increased, eventually one obtains a counting rate. The voltage at which the GM tube just begins to count is called the starting potential. The counting rate quickly rises as the voltage is increased. For our equipment, the rise is so fast, that the graph looks like a "step" potential. After the quick rise, the counting rate does not increase as fast as previously. This range of voltages is termed the "plateau" region. Eventually, the voltage becomes too high and we have continuous discharge. The threshold voltage is the voltage where the plateau region begins. Proper operation is when the voltage is in the plateau region of the curve. For best operation, the voltage should be selected fairly close to the threshold voltage, and within the first 1/4 of the way into the plateau region. In the plateau region the graph of counting rate vs. voltage is in general not completely flat. The plateau is not a perfect plateau. In fact, the slope of the curve in the plateau region is a measure of the quality of the GM tube. For a good GM tube, the plateau region should rise at a rate less than 10 percent per 100 volts. That is, for a change of 100 volts, (∆counting rate)/(average counting rate) should be less than 0.1. An excellent tube could have the plateau slope as low as 3 percent per 100 volts.

Quenching of GM tube:-. As we know that ions and electrons are generated in gas present

inside the GM tube. These ions and electrons move to opposite sides and reach the cathode and

anode completing circuit and producing current pulse. These ions and electrons, under the action of high voltage, can get a large kinetic energy and when they strike the cathode or anode ( for electron) they producing a sputtering effect. In effect it can eject electrons from cathode and these electrons and produces it own current. This process produces an unwanted current and in GM tube which make busy the counter and so next pulse is not recorded. To remove this process, heavy molecular mass molecules multi atom gas is added to GM tube. This gas is called quenching gas. This gas decrease the kinetic energy of ions striking the cathode and therefore sputtering process is avoided and this process is called quenching.

Dead time:-. The disadvantage of quenching is that for a short time after a discharge pulse has

occurred the tube is rendered insensitive and is thus temporarily unable to detect the arrival of any new ionizing particle. This effectively causes a loss of counts at sufficiently-high count rates and the time is called dead time T , which is typically a few microseconds. In count rates and calculation this time give some error so correction is needed. If C is total number of counts that we recorded, n is measured count rate with dead time present in counter and N is true counting rate when there is no dead time in counter. Then true count rate N is given as

Measuring the Resolving Time:-. We can get an estimate of the resolving time of our

detector by performing the following measurement. First we determine the counting rate with one source alone, call this counting rate n 1. Then we add a second source next to the first one and determine the counting rate with both sources together. Call this counting rate n 12. Finally, we take away source 1 and measure the counting rate with source 2 alone. We call this counting rate n 2. Ideally n 12 =n 1 +n 2. But this does not happen when dead time is present. The measured count rates are not equal but actual should be equal such as N 12 =N 1 +N 2. So we can obtain a relation using above formula.

Or

Procedure:-. Following procedure is followed.

1. Make arrangement as shown in figure.

Time Counts Rate(Gross)

Count rate (corrected)

Time Counts Rate(Gross)

Count rate (corrected)

Time Counts Rate(Gross)

Count rate (corrected) 10 652 640 170 125 113 340 64 52 20 501 489 180 106 94 350 63 51 30 408 396 190 116 104 360 68 56 40 369 357 200 103 91 370 55 43 50 229 217 210 94 82 380 52 40 60 195 183 220 104 92 390 54 42 70 144 132 230 92 80 400 45 33 80 146 134 240 85 73 410 54 42 90 121 109 250 80 68 420 49 37 100 97 85 260 85 73 430 51 39 110 92 80 270 76 64 440 47 35 120 97 85 280 71 59 450 45 33 130 108 96 290 71 59 460 44 32 140 89 77 300 67 55 470 36 24 150 73 61 310 75 63 480 35 23 160 100 88 320 56 44 490 41 29

Plots:

From this graph we can calculate slope of line 0.0043 which is decay constant and so half life is 161.197 second and source is Ag-108.

This long lived isotope is back extrapolated and so now we can get half life of short lived isotope.