1. PCS352 LAB #1 Characteristics of a Geiger Counter
In order to prepare for this first lab you should read and understand
Section 10.1 of Turner textbook
Lab procedure
These slides.
You have prepared well if you can answer all the questions posed at the end of this presentation

2. Ionizing radiation:
Some types of radiation have such high energy that they can break chemical bonds and set electrons free from atoms. When that happens the result is a pair (ip) ion-electron and the radiation is said to be ionizing radiation.
Since the energy needed for an electron to escape an atom is in the range 4 eV to 25 eV (depending on the atom) then, for radiation to be ionizing, it must carry energy in excess of this amount.

3. Types of Ionizing radiation:
g- rays: e.m. radiation emitted following nuclear transitions from higher lower energy states (~MeV)
X- rays: e.m emitted following electronic transitions in atoms (~keV)
Fast electrons (or positrons): b- (or b+) (if emitted from the nucleus)
d-rays: accelerated electrons (in accelerators) resulting from charged particle collisions
Heavy charged Particles: proton; alpha particle; nuclei of heavier atoms.
Neutrons: neutral particles produced in nuclear reactions or fission.

4. Terminology recommended by the ICRU Directly Ionizing Radiation:
(International Commission in Radiation Units and Measurements)
Directly Ionizing Radiation:
Fast charged particles which deliver their energy to matter directly though interactions along the particle’s track
Indirectly Ionizing Radiation:
Photon or neutrons which first transfer their energy to charged particles in the matter in a relatively few large interactions. These charged particles in turn deliver the energy to the matter as above.

5. Remember: Charged Particle Interactions
The figure on the left shows the path (often called “track”) of an alpha-particle in water. Each circle depicts an ionisation or excitation event. The branching tracks are "delta rays“ (electrons that result from the collisions).

6. Principle of functioning of “Gas filled” Detectors
The detection of radiation with “gas filled detectors” is based on the interaction of the radiation with the gas atoms causing electron-ion pairs to be produced.
Let us consider a very simple arrangement as shown in the figure: two parallel plates at a difference of potential V and with the space between them filled with an inert gas (argon or xenon).
Electric Field
Energy E I V + - A

7. Electric Field
Energy E I V + - A
A parallel beam of monoenergetic (E) charged particles enters the gas chamber across an area A .
As the particles slow down in the chamber they ionize gas atoms, ejecting electrons and leaving behind positive ions. The ejected electrons may immediately produce more electron-ion pairs.
Under the electric field created by the DC voltage V, the electrons will move towards the positive plate and the ions towards the negative plate.

8. Will these charges reach their respective electrodes?
Electric Field
Energy E I V + - A
Will these charges reach their respective electrodes?
It depends on the magnitude of the voltage V.
If V ~1mV, the electric field between the plates may be insufficient to move the electron and ion very far from each other and the two may recombine to re-form the gas atom.
If the V ~1MV it will likely get sparks flying between the two electrodes.
For an intermediate difference of potential V, the electron and ion will move to the respective plates without recombination or sparking occurring.

9. You will do that in your first lab.
Electric Field
Energy E I V + - A
All electron-ion pairs collected
Recombination occurs V0 I0
When the electron and ion move to the plates, an electric current is set up in the circuit and it can be measured with a meter. One can then plot the value of the current for different values of the voltage applied.
You will do that in your first lab.
I0 is the saturation current

10. This is a small pulse and requires an amplifier
Ionization Chambers are useful as devices to measure radiation exposure and radioactivity. They act also as a capacitor when they store energy.
Let’s say that an electron of energy 1 MeV enters and ionization chamber with a capacitance of 100 pF. If the energy required to ionize an atom of the filling gas is 30 eV, and the efficiency is100%, what voltage pulse would this electron produce ?
The increase in charge would then be:
If the capacitance, C, of the chamber is 100 pF then
This is a small pulse and requires an amplifier

11. The parallel plate design we consider before is a simplification of the gas-filled chamber which is generally cylindrical in shape with the positive electrode consisting of a thin wire running through the centre of the cylinder and the wall of the cylinder being the negative electrode, as illustrated below.
The electrons formed by the ionizing radiation are attracted to the central wire and flow through the resistor R causing a voltage pulse (V=RI).

12. When a charged particle enters the chamber, the potential difference across the plates drops momentarily while the ions are being collected. After collection it returns to the original value.
This voltage pulse that occurs during collection can be amplified and recorded electronically to register the particle. This voltage pulse can be amplified by some device such as a loudspeaker or a counter.
A voltage pulse is normally referred to as a pulse count and the number of counts per unit time called count rate.
Furthermore if the particle stops in the chamber, since the number of ions is proportional to the original energy of the particle, the size of each pulse can be used to determine the energy of the particle.
In addition to measuring absorbed energy, a parallel plate ionization chamber operated in the plateau region can also be used to count particles.

13. Graph of the dependence of the pulse height versus the DC voltage.
Region A: DC voltage is relatively low so that recombination of positive ions and electrons occurs. As a result not all ion pairs are collected and the voltage pulse height is relatively low. It does increase as the DC voltage increases since the amount of recombination decreases.
Region B: DC voltage is sufficiently high in this region so that only a negligible amount of recombination occurs. This is the region where a type of detector called the Ionization Chamber operates.

14. Graph of the dependence of the pulse height versus the DC voltage.
Region C: DC voltage is sufficiently high in this region so that electrons approaching the central wire attain sufficient energy between collisions with the electrons of gas atoms to produce new ion pairs. Thus the number of electrons is increased so that the electric charge passing through the resistor, R, may be up to a thousand times greater than the charge produced initially by the radiation interaction. This is the region where a type of detector called the Proportional Counter operates.

15. Graph of the dependence of the pulse height versus the DC voltage.
Region D: DC voltage is so high that even a minimally-ionizing particle will produce a very large voltage pulse. The initial ionization produced by the radiation triggers a controlled avalanche of electrons which head towards and spread along the central wire. This region is called the Geiger-Müller Region, and the Geiger Counter works in this range of voltage.because of the avalnche it does not need amplification
Region E: DC voltage is high enough for the gas to completely breakdown and thus is useless to detect radiation.

16. Features of “Geiger Counters”
Dependence on DC voltage – The plateau region is the zone of DC voltage operation. In these zone the voltage fluctuations will not affect the counting rate.

17. Do “Geiger Counters” need an amplifier? How is the avalanche stopped?
A Geiger Counter operates at relatively high DC voltages (for example volts) such that an avalanche of electrons is generated following the absorption of radiation in the gas. Because of the avalanche an amplifier is not needed.
How is the avalanche stopped?
The process of stopping the electron avalanche is called Quenching.
It can be done by electronically lowering the DC voltage following an avalanche.
But a more widely used method of quenching is by using a quenching gas, which is, for example, ethyl alcohol in vapour form. The quenching gas consists of relatively large molecules and their role is to absorb the energy which, in their absence, would give rise to sustaining the electron avalanche.
The large molecules of the quenching gas act like a brake to the chain effect.

18. Dead Time in “Geiger Counters”
Dead Time – is the period of time following the entrance of radiation during which the detector is insensitive to radiation, (regardless of the type of quenching used) and as a result the reading obtained with this detector is less than it should be.
Dead times are relatively short - typically of the order of µs. One can use the following formula to determine the true count.
T is the true counting
A is the measured counting reading
t is the resolving time which equals the sum of dead time and recovery time.

19. Answer the following questions in a brief, concise, complete and accurate manner. The lab quiz will address questions like the ones given here.
1. What is the difference between a proportional counter and a Geiger counter?
2. Does a Geiger counter need an amplifier? Explain your answer.
3. What is the role of a quenching gas?
4. Why does a Geiger counter operate in the plateau region?
5. Define Dead Time, Recovery Time and Resolving time for a Geiger counter.
6. A Geiger counter with a Resolving Time of 300 ms is used to detect radiation from a radioactive source. If the observed count rate is 1100 s-1, what is the true count rate?
7. A Geiger counter measures two sources separately and together in order to calculate the resolving time. If the average counting rates per minute obtained are; 346 with source A, 255 with source B and 600 with both sources, what is the Resolving Time (don’t forget the units)?
8. Derive equation 2. in the lab procedure.