Geiger Muller Tube and Radio Carbon Dating

Geiger-Muller Tube

A Geiger-Muller (GM) tube is a device used to detect ionizing articles or nuclear radiation. It consists of a glass cylinder G coated with a metal inside it as cathode C. A metal wire runs along the axis of the cylinder as anode A. as shown in the figure. The cylinder is filled with a gas such as argon mixed with halogen vapour at a low pressure of about 10 cm of Hg. The tube has very thin mica end window W shielded by a protective gauge P.

The high tension source (H.T.) supplies a potential difference of value little less than the ionizing potential of the gas through the resistance R. when a single ionizing particle, like a β particle or a γ-ray photon, enter the tube, through the window W, it will produce the ionization of the gas. The potential applied accelerates the ions towards the electrodes which produce more ions making collisions with gas molecules. So, an avalanche of ions is produced inside the tube which rushes towards the electrodes and produces a current in external resistance R. The resulting drop in potential across R reduces the potential across the tube below the threshold value and the decays rapidly as the circuits have a small time constant. This momentary surge of current produces an electric pulse which corresponds to one particle entering the counter. The successive current pulse generated in the resistance R, corresponding to the number of ionization particles passing into the tube, are detected and counted by a suitable arrangement.

The radioactive particles emitted from a given sample in a given interval of time can be counted by placing the sample at some distance from the window.

Radio Isotopes

The isotopes of an element which are also radioactive are called radioactive isotopes or simply radioisotopes. For example, C¹⁴, C¹² C¹¹ are the isotopes of carbon. As C¹⁴ and C¹¹ emit radioactive radiation, they are known as radioisotopes of carbon. Similarly, Na²³, Na²⁴ are the radioisotopes of sodium.

Use of Nuclear Radiation

Nuclear radiations are widely used in medicine and therapy.

1. Diagnosis
   1. Radioiodine I¹³¹ is used to determine the condition of the human thyroid gland.
   2. Radio sodium (Na²⁴) is used to locate the place of construction, restricting blood circulation in a patient.
   3. Radio chromium (Cr⁵¹) is used to locate the exact position where hemorrhage might have taken place inside the body.
   4. Water contents of the body are measured by using deuterium and tritium as the tracer.
   5. Ga⁶⁷ is used in tumour localization.
   6. Tc⁹⁹ is used in locating cancers, embolism, and other pathologies.
2. Therapy
   1. Gamma radiations emitted by radiocobalt (Co⁶⁰) are used destroy concerns tumors in the body.
   2. Radio gold (Au¹⁹⁸) and radiocobalt are used in a treatment of some cancer.
   3. Radio phosphorous (p³²) is used for treating skin diseases and leukemia.
   4. Radio bismuth is use in the treatment of syphilis.
   5. Radioiodine is used to treat an overactive thyroid gland.
3. Gamma radiations from cobalt-60 are used in hospitals for sterilization of materials like dressing, hypodermic syringes, and surgical sutures. Heat sensitive instruments and materials are sterilized by gamma radiations.
   1. Many radiations from cobalt-60 are used for industrial radiography i.e. for investing the interiors of metallic castings for detecting any flaws or defects.
   2. Radioactive radiations are used in the production and modification of plastic.
   3. Nuclear radiations like γ-rays have been utilized for a preservation of food.
   4. Radiation mutation in plants has been practiced to produce new varieties of these plants.
   5. Radioactive iron can be used to study piston ring were.
   6. The effectiveness of detergent products is tested using radioactive dirt.

Nuclear Reaction

The reaction involving the change in the structure of nucleus itself is called nuclear reaction. The first artificial nuclear reaction was produced by Rutherford in 1919. The reaction is

$$_2He^4 + _7N^{!4} \rightarrow _8O^{17} + _1 H^1 $$

In this reaction ₂He⁴ is bombarding particle, ₇N¹⁷ is target nucleus, ₁H¹ is product particle and ₈O¹⁷ is product nucleus. The reaction can be written as \(\alpha (N^{14}, O^{17} ) P\). In all nuclear reaction, conservation principle for a charge, momentum, angular momentum, mass-energy, mass number etc are obeyed.

Radio Carbon Dating

The process of dating an ancient object by measuring the proportion of ₆C¹⁴ to ₆C¹² in a specimen is called carbon dating.

The atmosphere contains carbon ₆C¹² and its radioisotopes ₆C¹⁴. Neutron produced by cosmic rays reacts with nitrogen in air to form ₆C¹⁴ as below:

$$_7N^{14} + _0n^1 \rightarrow _6C^{14} + _1 H^1 $$

The ratio ₆C¹⁴ to ₆C¹² in the atmosphere is about 10^-12. Both ₆C¹⁴ to ₆C¹² combine with oxygen to form carbon dioxide that is absorbed by the living plants during photosynthesis process. But ₆C¹⁴is a β-emitter and decays back into nitrogen as

$$_6C^{14} \rightarrow _7N^{14} + _{-1} e^0$$

As the living planet performs photosynthesis, the decay of C¹⁴ is compensated by a new supply of it from atmosphere air and a radioactive equilibrium is reached where there is a fixed ratio of ₆C¹⁴ to ₆C¹².

Suppose the activity of some dead material was R₀ at the time of death and it has been reduced to R after t years. Then, according to law of radioactive decay, we have

\begin{align*} R &= R_0 e^{-\lambda t} \\ \text {or,} e^{-\lambda t} &= \frac {R_0} {R} \\ \text {Taking log on both sides } \\ \text {or,} \lambda t &= \log _e \left (\frac {R_0}{R} \right ) \\ \text {or,} t &= \frac {1}{\lambda } \log _e \left (\frac {R_0}{R} \right ) \\ \therefore t &= \frac {1}{\lambda } \log _e \left (\frac {R_0}{R} \right ) \\ \text {But} \: \lambda = \frac {0.693}{T} ,\ \end{align*} where T = 5730 years, the half of the radioactive carbon. So\begin{align*} \\ t &= \frac {T}{0.693 } \log _e \left (\frac {R_0}{R} \right ) \\ \end{align*}

Hence by measuring the activity R₀ of living plant and activity R of its dead material, the age t of that object can be determined.

Health Hazards and Safety Precautions

Following types of damages can be caused by the radiation hazards:

1. The exposure to radiation induces deleterious genetic effects.
2. The strong α-ray exposure can cause lung cancer.
3. The exposure to fast and slow neutron can cause blindness.
4. The exposure to neutrons, protons and α-particle can cause damage to red blood cells.
5. The strong exposures to protons and neutrons can cause serious damage to reproductive organs.

Following are safety precautions for radiation hazards

1. The radioisotopes should be transferred in thick walled lead containers and kept in rooms with thick walls of lead.
2. The radioisotopes are handled with the help of remote control device.
3. The workers are asked to wear lead aprons.
4. The radioactive contamination of the working area is avoided at all costs.
5. Nuclear explosions should be carried out far away from the public area.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, Bhesha Raj Adhikari, Arjun Kumar Gautam, Parashu Ram Poudel. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.