Counter – 3

1. Counter – 3

2. Gas counter • A gas-filled chamber has a central electrode (anode, electrically positive) and a conducting wall (cathode, negative). • They are maintained at different potential. • If a charged particle or gamma ray is allowed to enter the chamber, it may produce a certain amount of ionization in the gas.

3. The resultant positive ions and electrons are attracted toward the negative and positive surfaces, respectively. • A charge moves in the local field E with a drift velocity vD = µE, where the mobility µ depends on the time between collisions and the mean free path.

4. If a magnetic field is present, charges tend to execute circular paths interrupted by collisions. • When the voltage across the tube is low, the charges merely migrate through the gas, they are collected, and a current of short duration (a pulse) passes through the resistor and the meter.

5. More generally, amplifying circuits are required. • The number of current pulses is a measure of the number of incident particles that enter the detector, which is designated as an ionization chamber when operated in this mode.

6. If the voltage is then increased sufficiently, electrons produced by the incident radiation through ionization are able to gain enough speed to cause further ionization in the gas. • Most of this action occurs near the central electrode, where the electric field is highest.

7. The current pulses are much larger than in the ionization chamber because of the amplification effect. • The current is proportional to the original number of electrons produced by the incoming radiation, and the detector is now called aproportional counter.

8. If the voltage on the tube is raised still higher, a particle or ray of any energy will set off a discharge, in which the secondary charges are so great in number that they dominate the process. • The discharge stops of its own accord because of the generation near the anode of positive ions,  which reduce the electric field there to such an extent that electrons are not able to cause further ionization.

9. … • The current pulses are then of the same size, regardless of the event that initiated them. • In this mode of operation, the detector is called a Geiger-Mueller (GM) counter.

10. Neutron detector • To detect neutrons, which do not create ionization directly, it is necessary to provide a means for generating the charges that can ionize a gas. • n absorption in boron: 0n1 + 5B10 2He4 + 3Li7

11. One form of boroncounteris filled with the gas boron trifluoride (BF3) and operated as an ionization chamber or a proportional counter.

12. SCINTILLATION COUNTERS • The name of this detector comes from the fact that the interaction of a particle with some materials gives rise to a scintillation or flash of light. • The basic phenomenon is familiar — many substances can be stimulated to glow visibly on exposure to ultraviolet light.

13. Molecules of materials classed as phosphors are excited by radiation such as charged particles and subsequently emit pulses of light. • The substances used in the scintillation detector are • inorganic (e.g., sodium iodide or lithium iodide) or • organic, • in one of various forms — crystalline, plastic, liquid, or gas.