Geiger Counter

Introduction

In this experiment a Geiger counter was used to measure ionizing radiation. Then, the data obtained from the experiment was used to determine the characteristic curve for Geiger counter. The experiment was designed to examine the intensity of radiation as a function of distance from source. Strontium-90, a beta emitter was used as the radioactive source which has a half-life of 28 years. The data obtained from the Geiger counter about the radioactive decay was used to create graphs so that the information can be better analyzed.

Theory

The Geiger counter uses the ionizing property of radiation to detect the kind of radioactive emissions. Decay reactions involve one of the four types of processes: alpha decay, beta (+ or -) decay, electron capture and gamma decay. The radioactive decay of 90Sr38 is an example of beta minus decay. Beta particles are electrons or positrons. In beta mius decay a neutron is converted into a proton while an electron and an anti-neutrino are emitted. The parent nuclide retains the same mass number and increases its charge number by one as it changes into the daughter nuclide. The equation involved in the beta-minus decay of Sr is as follows:

         β minus decay

90Sr38                90Y39 + 0e-1 +ν[pic 1]

In the Geiger counter, the ionizing radiation passes through the metal cylinder into the tube where it ionizes argon atoms. The freed electrons are then attracted toward the positive wire, which are then accelerated and given enough energy to collide with other Argon atoms and ionize then on the way to the wire. These ionizations cause avalanche discharge and current is produced in the circuit. The lowest voltage for which radiation can be detected is known as the starting voltage or threshold voltage. As the tube voltage is increased above threshold voltage, the count rate increases. At the operating voltage the count rate N will depend on the number of particles per unit time entering the window area of the tube. For a point source located at distance r from the tube, which emits equally in all directions (isotropic source), the count rate by the Geiger tube should be inversely proportional to r2.