El arte, mediador de la formación integral

Radiation has an ability to ionize gases.

Radioactive substances emit 3 kinds of radiation:

(a) Alpha(α) –rays (b) Beta(β) –rays (c) Gamma(γ) –rays

Detection of Radioactivity 1. Gold-leaf electroscope

As a radium source is brought near cap of negatively-charged electroscope, radiation emitted by radium source ionizes air molecules above cap. As cap is negatively-charged, negative ions are repelled while positive ions are attracted to cap. These ions neutralize negative charges on the cap and the gold leaf thus collapses.

2. Diffusion cloud chamber

Air containing alcohol vapour in a chamber is cooled with dry ice placed below a thin black metal plate. When radioactive source is introduced into the chamber, the radiation produced passes through the vapour leaving white tracks on black plate in the dense vapour due to condensation of alcohol vapour on ions formed.

The table shows how the tracks tell what kind of radiation was introduced.

Radiation Tracks made Characteristics

α–particles Tracks are straight, short and thick; proving that radiation is strongly ionizing

β–particles

Tracks are twisted, thin and long; proving that the radiation is less ionizing than α–particles.

The twisted nature is because β–particles are easily deviated by collisions with vapour molecules.

γ–particles Tracks are short, thin and irregular; proving that the radiation is least ionizing.

3. Geiger-Müller (GM) tube

When ionizing radiation enters the tube by penetrating the thin mica window, argon atoms will ionize to electron and argon ion pairs. The free electrons will accelerate towards fine wire anode placed parallel between 2 cylindrical cathodes. The accelerating electrons will cause further ionization of argon atoms by colliding with them, producing many electrons collected on anode. Positively-charged argon ions will attract towards cathode and the collection of electrons and argon ions at the electrodes produces pulse which is amplified and fed to a ratemeter (Refer OCR for link of pulse) which has grids marked in counts per second from which average pulse rate can be read.

When the radioactive source is removed, a continuing register but low pulse rate is read on the GM tube, which is called background count caused by background radiation.

Background radiation is caused by contamination of detector or its surrounding;

or by the cosmic radiation entering Earth atmosphere from outer space. In experiment, we omit the low reading of background count.

The characteristics of 3 kinds of radiation

Types of Radiation α-particles β-particles γ-particles Nature of radiation Positively-charged Ionization is the removal of electrons from a neutral atom to dissociate into electrons and positively-charged ions.

23.2 Half-Life Radioactive Decay

It is the process when a group of unstable nuclei disintegrate to become more stable.

Since it is not affected by chemical combinations or external conditions, radioactive emissions occur randomly over space and time, i.e. we cannot predict which nucleus and when electrons will disintegrate.

To show that radioactive emission occurs randomly over space

Position a few GM tubes, all equidistant from a radioactive source. The count rates on each GM tube will not be the same.

To show that radioactive emission occurs randomly over time

Place a GM tube near a radioactive source with long half like and determine the disintegration over a minute, which will tell us count rate. Repeat experiment a few times and since radioactive has long half life, the count rate should be same but the readings show slight fluctuation.

Half-Life

Half-life is the time taken for half of the unstable nuclei to decay.

Let‟s compare ten million radioactive sodium nuclei with half-life of 15 hours with ten million radioactive radium nuclei with half-life 1600 years. It will take 15 hours for 5 million sodium nuclei to decay but 1600 years for radium nuclei to decay (half the amount)

The table shows sample count rate of a radioactive substance. Half-life is 7.5 hours.

Count rate/min 5000 2500 1250 625 312.5

Time/h 0 7.5 15 22.5 30

26.3 Radiation: Applications, Hazards, Precautions The applications

1. Tracers

The ability of detectors to measure small concentration radioactive material can be used to:

- Find out the function of thyroid as the rate of radioactive iodine-131 applied on the thyroid to accumulate in it.

- Find torn parts in moving components of machinery by applying radioactive isotope on surfaces of moving parts to find out how much of the radioisotope is rubbed off.

- Find leaks in underground pipes as leaks emit an unusually high count rate on GM detector at area of leak.

- Find how well plants absorb phosphate by radioactive phosphorus-32.

2. Penetrating radiation

- Gamma rays can photograph deep inside engine to check any faults.

- Gamma rays can be used to check constant thickness of rolled metal sheets.

The rays is radiated from a source at one side of the moving sheets and on the other side, there‟s a ratemeter to find out count rate which depends on amount of radiation passing through steel plates. When plates are thick, low count rate and vice versa. The count rate is constant when the steel plates have equal thickness.

- High penetrating power of gamma rays is used to kill bacteria in frozen or pre-packaged foods to sterilize food and prevent food poisoning.

3. Power sources

- Uranium-235 is used as fuel in nuclear power stations (Refer chapter 24)

- Some fire alarms emit α-particles to keep air around them slightly ionized so that any changes in level of ionization caused by smoke can be detected and the alarms go off.

4. Medical uses

Gammatron decays radioactive cobalt-60 to emit β-particles and γ-rays. When properly shielded, γ-rays can be brought to bear on deep cancerous growths in a cancer patient and the radiation kills the cells of tumor.

5. Archaeological dating

Radioactive carbon-14 isotope is present in air. When animals breathe in these, they become slightly radioactive. When they die, the carbon inside them will start to decay. The half-life of carbon-14 is almost 5500 years, so the age of dead animals can be found by comparing activity of carbon-14 in dead animals with a living one. The activity of the carbon in living animals is constant as it‟s continuously replenished while the carbon in dead animals is not replenished.

The hazards 1. Overexposure

- Radioactive radiation overexposure result in radiation burns, lead to sores &

blisters for long time. Sometimes, this cause radiation sickness leading to death.

- Radioactive radiation can lead delayed conditions, e.g. eye cataracts/leukemia may appear many years later.

2. Genetic mutations

- The ionizing radiation cause genes to be destroyed or mutated leading to offspring with physiological and other abnormalities.

3. Radioactive leakage

- Accidents which may cause leakage of radioactive materials into the air can pose health problems to people, livestock and plants.

The Precautions

To prevent overexposure/accidents, the following measures must be taken:

(i) Workers working with γ-rays must wear film badges or pocket dosimeters to keep track of accumulated dose of radiation they are exposed at a time.

(ii) Always keep radioactive sources in lead-lined boxes kept in storage rooms built with lead bricks of 1m thick labeled with “Radioactive Material” as radioactive radiation do not penetrate thick lead.

(iii) Radiation symbol must be displayed whenever radioactive radiation experiment is conducted.

(iv) Persons doing experiments should use special protective coating such as lead-lined suits and lead-lead-lined gloves, holding the radioactive source with tweezers.

At the end of the experiment, the contaminated clothing MUST be changed.

(v) Food and drinks are prohibited when radioactivity experiment is made as radioactive dust contaminating food may be taken into the body.

EXERCISE

Below is half-life curve for mercury. The count rate is given in percentage.

(a) Calculate the half-life of mercury-203

(b) 120g of this mercury sample was left from January 1 until June 30. What is the approximate mass of mercury on June 30 as it decays?

(c) Will the mercury be totally used up over time? Explain your answer.

Two radioactive radiations, alpha-particles and gamma-rays, are emitted from a radioactive source. Explain

(a) how a Geiger-Muller tube calculates the count rate of this radiation.

(b)how you would prove that the radioactive radiation emitted were alpha-particles and gamma-particles.

A 200 g sample of lawrencium is left in a container from 8:00 AM one morning until 2:00 PM the next afternoon. If the mass of the sample was one-eighth its initial mass, what is the half-life of lawrencium?