Radioactivity

Question 1

Who discovered radioactivity?

Answer 1

Henri Becquerel (in 1896)

Question 2

Describe the first experiment that proved naturally occuring spontaneuous radioactivity.

Answer 2

Beqcquerel covered a photographic plate with opaque paper and left uranium salts near it and found that the plate became fogged as if it was partially exposed.

Question 3

What is radioactivity?

Answer 3

Radioactivity is essentially energy being emitted from overactive elements. The nuclei of these elements have too much energy and get rid of it through radiation.

Question 4

What are the three types of radiation?

Answer 4

• Alpha radiation
• Beta radiation
• Gamma radiation

Question 5

State the characteristics (structure, relative charge, mass) of alpha radiation.

Answer 5

Structure - Helium nucleus
Relative Charge - +2
Relative Mass - 4

Question 6

State the characteristics (structure, relative charge, mass) of beta radiation.

Answer 6

Structure - Electron
Relative Charge - -1
Relative Mass - Approximately 0

Question 7

State the characteristics (structure, relative charge, mass) of gamma radiation.

Answer 7

Structure - Energy (EM)
Relative Charge - 0
Relative Mass - 0

Question 8

Describe what happens during alpha decay.

Answer 8

When a radioactive element emits an alpha particle it will lose 2 protons and 2 neutrons. Due to this the atomic number decreases by 2 and the mass number decreases by 4.

Question 9

Describe what happens during beta decay.

Answer 9

A neutron breaks into a proton and electron. The electron is ejected from the nucleus as beta radiation. Due to this the atomic number increases by 1.

Question 10

Describe what happens during gamma decay.

Answer 10

Gamma decay is EM radiation. It is an example of pure photon energy being emitted to get rid of excess energy in the nucleus. It will have no effect on mass or atomic number. Gamma radiation usually occurs in conjunction with the other decay types.

Question 11

State the penetrating power, ionising ability and rate of deflection in EM fields of the types of radiation.

Answer 11

Name - Penetrating Power - Ionising Ability - Deflection
Alpha - Lowest - Greatest - Behaves as a positive particle
Beta - Medium - Medium - Behaves as a negative particle
Gamma - Greatest - Lowest - No deflection

Question 12

How would you demonstrate penetration ability?

Answer 12

Geiger counter, radiation sources and material.

Question 13

How would you demonstrate ionising ability?

Answer 13

A negatively charged electroscope.

Question 14

How would you demonstrate deflection in an EM field?

Answer 14

Radiation source, charged plates, fluorescent screen.

Question 15

What is a radioisotope?

Answer 15

Any isotope that decays and emits radiation.

Question 16

What can radioisotopes be used for?

Answer 16

• Food irradiation
• Medical uses
• Carbon dating
• Non-destructive testing
• Smoke detectors

Question 17

Describe what is meant by food irradiation.

Answer 17

Food can be irradiated by exposing it to an isotope such as cobalt-60. The energy from the ray passing through the food can destroy many bacteria that cause disease or food spoiling. The radiation does not change the taste or texture, and the food is safe to eat.

Question 18

What medical uses come from radioisotopes?

Answer 18

All three radiation types can be used to treat different cancers. Gamma radiation is also used to sterilise instruments which would otherwise melt at high temperatures. Tracers are also used to show pathways within the body. Alpha radiation is not ingested as it would remain in the body.

Question 19

Explain carbon dating.

Answer 19

The carbon-14 isotope is created by cosmic radiation bombarding the nitrogen in the atmosphere. This becomes part of the plant-animal cycle through photosynthesis. Therefore the ratio of carbon-14 to carbon-12 in living plants and animals is roughly the same as in the atmosphere. When an organism dies, this ratio changes as the carbon-14 decays to carbon-12. We can judge the rough age of the specimen by measuring the amount of carbon-14.

Question 20

How can radioisotopes be used for non-destructive testing?

Answer 20

Gamma and beta radiation can be used to check for welding faults or structure thickness.

Question 21

How can radioisotopes be used for smoke detectors?

Answer 21

Alpha radiation can be used in smoke detectors. By placing a weak alpha source near two electrodes, current can flow as air is ionised. If smoke particles become too dense, the current stops and an alarm will go off.

Question 22

What are some methods of detection of radiation?

Answer 22

• Geiger-Müller tube
• Solid state detector

Question 23

Explain how the Geiger -Müller tube works.

Answer 23

The G-M tube can be used to detect the level of alpha, beta and gamma radiation, however does not identify the type. It works on the principle of ionisation of gas particles. It consists of a container with a mica window at one end through which radiation can pass. Inside the container is low-pressure argon gas, a cylindrical cathode and anode rod. Micais used an it has a low mass per unit area and allows low-penetration ionising radiation to pass easily. There are two electrodes between which there is a potential difference of a couple hundred volts. As radiation enters, some molecules of the gas are ionised which creates positively charged ions and electrons, known as ion pairs. The electric field created by the electrodes accelerates the positive ions towards the cathode and the electrons towards the cathode. This causes a current and the amount of current is converted to pulses or counts by an external amplifier and counter. At a certain point, an avalanche effect occurs in which travelling ions collide with other particles and cause further ions. This increases the count rate.

Question 24

Explain how a solid state detector works.

Answer 24

A solid state detector detects the amount and type of radiation by using a semiconductor p-n junction. Radiation coming into the detector hits the depletion layer. Electron-hole pairs are produced, which allow current to flow. Like in a G-M tube, the amount of current is converted to counts by an external amplifier and counter.

Question 25

What is activity/the rate of decay?

Answer 25

The number of nuclei of a radioactive source that decay every second. Measured in Bequerels.

Question 26

What is the law of radioactive decay?

Answer 26

The activity of a radioactive sample is always proportional to the number of nuclei that remain undecayed.

Question 27

What is half-life?

Answer 27

The time taken for half of the nuclei of a radioactive source to decay.