0625 · 5.2Radioactivity

Detecting background radiation and count rate

0:00A smoke alarm can use radiation to help protect people from fire. The same broad phenomenon can damage living cells or help doctors treat cancer. The difference depends on the radiation, its source and how exposure is controlled. By the end of this video, you will understand detection, radioactive decay, half-life, useful applications and safety. You need a little knowledge about atoms before we begin. A nucleus contains positively charged protons and uncharged neutrons.

0:27The proton number identifies the element, while the nucleon number counts protons and neutrons together. Isotopes are atoms of the same element with different numbers of neutrons. Radioactivity involves changes inside an unstable nucleus, rather than changes in the surrounding electrons. Ionising radiation has enough energy to remove electrons from atoms, producing ions. This explains both how some detectors work and why exposure can damage cells. We will begin with detecting radiation that is already around us.

0:58Background radiation means the ionising emissions present in our surroundings all the time. A detector can register it even when no radioactive source has deliberately been placed nearby. This does not mean the detector is faulty. It means there are natural and other sources of ionising radiation around us. Radon gas in the air makes a significant contribution to background radiation. It forms through radioactive decay in rocks and soil, and can enter buildings.

1:22Rocks and building materials also contain naturally occurring radioactive isotopes. Food and drink contain small amounts of radioactive substances, while cosmic rays reach us from space. Ionising nuclear radiation can be measured using a detector connected to a counter. A common example is a Geiger–Müller tube, in which incoming radiation can ionise gas. The detector produces an electrical pulse that the counter records. A recorded count indicates a detection, not necessarily every decay occurring inside the source.

1:53Count rate means the number of counts recorded per unit time. We measure it in counts per second or counts per minute. Count rate equals the number of counts divided by the measurement time. Longer measurements help reduce the effect of random fluctuations, although individual readings will still vary. For example, suppose a detector records six hundred counts in two minutes. Dividing six hundred by two gives three hundred counts per minute.

2:18Two minutes is also one hundred and twenty seconds. Dividing six hundred by one hundred and twenty expresses the same measurement as five counts per second. For Supplement, or Extended, you must determine a corrected count rate. First measure the background without the source, then measure again with the source present. The second reading includes both contributions. To find the corrected count rate, subtract the background from the measured total, keeping both in the same units.

2:45Suppose the background produces one hundred and twenty counts in two minutes. Dividing one hundred and twenty by two gives sixty counts per minute. With the source present, the measured count rate is four hundred and sixty counts per minute. Subtracting sixty gives a corrected count rate of four hundred counts per minute.

Alpha, beta and gamma properties

3:02Radiation emitted from a nucleus is spontaneous and random in direction. Spontaneous means the emission happens without an external trigger. Random direction means we cannot predict which way an individual emission will travel. The three types you need to recognise are alpha particles, beta particles and gamma radiation. An alpha particle is a helium nucleus, containing two protons and two neutrons. It has a charge of plus two and a relatively large mass.

3:28A beta particle is a fast electron emitted from the nucleus. Here, beta always means negatively charged beta emission, not positron emission. Gamma rays are electromagnetic waves emitted from the nucleus. It has no electric charge and is not a collection of protons or neutrons. Alpha and beta emissions involve particles with mass. Gamma emission carries energy away as electromagnetic radiation, rather than removing nucleons from the nucleus. Ionising ability describes how strongly radiation produces ions as it passes through matter.

4:00Alpha is the most strongly ionising of the three types, followed by beta, with gamma the weakest. Penetrating ability describes how far radiation can pass through materials. Strong ionisation generally means energy is transferred rapidly along the path. Alpha particles are stopped by paper or a few centimetres of air. Beta particles penetrate paper, but a few millimetres of aluminium stop them. Gamma radiation is the most penetrating, and thick lead greatly reduces its intensity.

4:29Avoid claiming that a thin lead sheet stops all gamma radiation, because shielding only reduces its intensity. The Extended explanation links ionisation to kinetic energy and electric charge. Alpha and beta particles carry kinetic energy that can be transferred to atoms. Their charges allow electrical interactions with electrons in the material. Removing an electron requires energy, so repeated ionisation reduces the kinetic energy of the travelling particle. An alpha particle has twice the charge magnitude of a beta particle and produces dense ionisation along its short path.

5:02It transfers kinetic energy rapidly, explaining its low penetration. Beta particles produce fewer ions per unit distance and usually travel further. Uncharged gamma radiation interacts less frequently, making it less ionising and more penetrating. Deflection in fields is also Extended content. In an electric field, positive alpha particles bend towards the negative plate. Negative beta particles bend towards the positive plate, so their deflection is in the opposite direction. Beta particles are deflected more because their mass is much smaller; gamma radiation is not deflected.

5:36A magnetic field can also deflect moving charged particles travelling across it. Alpha and beta particles curve in opposite directions because their charges have opposite signs. In the standard comparison, beta particles curve more strongly than alpha particles. Gamma radiation follows a straight path because it has no electric charge.

Radioactive decay and nuclear equations

5:56Radioactive decay is a change in an unstable nucleus that can result in alpha or beta emission, or gamma radiation. These changes are spontaneous and random. Random decay means we cannot predict when a particular nucleus will decay. However, a large collection of nuclei shows a predictable overall pattern. Random does not mean that all nuclei decay after waiting for the same time. Some may decay almost immediately, while others remain unchanged much longer.

6:22Spontaneous does not mean that heating or pressing the sample starts the process. Ordinary changes in temperature or pressure do not significantly alter radioactive decay. For Extended, some isotopes are radioactive because they have excess neutrons or because their nuclei are too heavy. The balance between neutrons and protons affects nuclear stability. A very heavy nucleus may also be unstable. Decay moves the nucleus towards greater stability, although the resulting nucleus may still be radioactive.

6:52During alpha decay, the nucleus loses two protons and two neutrons. Its nucleon number decreases by four, while its proton number falls by two. Because the proton number changes, the daughter nucleus belongs to a different element. For a heavy unstable nucleus, losing this cluster can increase stability. Decay equations using nuclide notation are Extended content. The upper number represents the nucleon number, and the lower number represents the proton number.

7:20Uranium two hundred and thirty-eight decays into thorium two hundred and thirty-four by emitting an alpha particle. The total upper and lower numbers must balance. For the upper numbers, 234 + 4 = 238. For the lower numbers, 90 + 2 = 92. This gives a useful check on the equation. It also explains why simply subtracting four from both numbers would describe the wrong nuclear change. During beta decay, a neutron changes into a proton and an electron.

7:48The electron is emitted from the nucleus as the beta particle. It does not come from the shells around the nucleus. The nucleus now contains one fewer neutron and one more proton, reducing an excess of neutrons. The nucleon number stays unchanged because a neutron has simply become a proton. The proton number increases by one, so beta decay also produces a different element. Carbon fourteen therefore becomes nitrogen with the same nucleon number.

8:15Its proton number rises from six to seven, while the emitted electron is written with a lower value of negative one. The beta equation balances because 7 + −1 = 6. The electron has a nucleon number of zero because it contains no protons or neutrons. Its lower value of negative one represents its charge, not a count of protons. This notation keeps the charge balance visible in the equation.

8:39Gamma emission removes excess energy from a nucleus, often following alpha or beta decay. The proton number and nucleon number both remain unchanged. The nucleus therefore remains the same element and isotope, but has lower energy. Unlike beta decay, gamma emission does not reduce the number of excess neutrons.

Half-life and corrected count rates

8:57The half-life of a particular isotope is the time taken for half the nuclei of that isotope in any sample to decay. After one half-life, half the original nuclei remain undecayed. After another, half of those remaining nuclei survive. Each equal interval removes the same fraction, not the same number. Suppose a sample initially contains eight thousand undecayed nuclei, and its half-life is three hours. After three hours, 8000 ÷ 2 leaves four thousand.

9:25After six hours, 4000 ÷ 2 leaves two thousand. After nine hours, 2000 ÷ 2 leaves one thousand undecayed nuclei. Corrected count rate also halves over each half-life when the measurement arrangement stays unchanged. Fewer undecayed nuclei remain, so fewer emissions occur per second on average. Individual measurements fluctuate because decay is random. We therefore look for the overall decay pattern, rather than expecting every reading to fit perfectly. For a graph with background already removed, choose a point on the smooth decay curve.

9:58Halve its count rate, then find where the curve reaches that lower value. Read the difference between the two times. Here, the rate falls from one hundred to fifty counts per second in four minutes, which is the half-life. Extended questions can give readings from which background has not been subtracted. In this example, background is twenty counts per second and the initial total is one hundred and eighty. Subtracting twenty gives an initial corrected rate of one hundred and sixty counts per second.

10:26Halving that corrected rate gives eighty counts per second. To locate that level on the original graph, add the background back. 80 + 20 gives a total reading of one hundred counts per second. The curve reaches this value after five minutes, which is the half-life. At ten minutes, the total is sixty, corresponding to a corrected rate of forty. A common mistake is halving the total reading of one hundred and eighty directly.

10:50That gives ninety, but background does not halve with the source. Another mistake is treating half-life as the time until every nucleus has decayed. The definition concerns half the nuclei, and the curve does not suddenly end after two half-lives.

Radiation applications, risks and safety

11:04Choosing isotopes for applications is Extended content, and involves both radiation type and half-life. Penetration determines whether emissions reach the intended target or detector. Ionisation determines their effect on the surrounding material. Half-life determines how quickly the source weakens and how long a useful or unwanted exposure can continue. An ionisation smoke alarm uses an alpha source to ionise air between electrodes. The ions allow a small current to flow.

11:32When smoke enters, the current falls and the alarm sounds. Alpha radiation is strongly ionising and has low penetration, while a long half-life keeps the source effective for many years. Gamma radiation can irradiate food to kill bacteria and sterilise medical equipment. Its penetrating ability allows it to reach microbes within food or through suitable packaging. Ionisation damages the microbes, reducing infection or spoilage. A source with a long half-life keeps working for years.

12:02The treated food and equipment do not become radioactive. Thickness control uses a source on one side of a material and a detector on the other. A thicker sheet absorbs more radiation, so the detected count rate falls. A thinner sheet absorbs less, so the reading rises. The control system adjusts the rollers using this change in detector signal. Beta radiation is useful for monitoring thin paper or aluminium because some passes through and some is absorbed.

12:29Alpha would usually be stopped completely, while gamma might pass through with too little change. Gamma can suit thicker metal, where greater penetration is needed. A long half-life prevents rapid source weakening from being mistaken for increased thickness. For diagnosis, a gamma-emitting tracer can help locate disease, including some cancers. Gamma radiation escapes the body so an external detector can locate it. A short half-life limits exposure after the investigation, but must allow enough time for measurements.

12:57The tracer must also reach the relevant tissue for the image to be useful. Gamma radiation can also treat cancer by killing cells in a tumour. Its penetration allows an external source to reach tissue inside the body. Carefully directed beams reduce unnecessary exposure to surrounding healthy cells. An external treatment source needs a practical working lifetime, unlike a diagnostic tracer placed inside the patient. Ionising nuclear radiation can cause cell death, mutations and cancer.

13:26Ionisation can damage important molecules inside cells, including their genetic material. A mutation is a change in that genetic material. Some damage kills a cell, while other damage may allow uncontrolled cell division that leads to cancer. Low penetration does not mean that alpha sources are always harmless. Outside the body, the outer skin stops alpha particles. If radioactive material is inhaled or swallowed, emissions can transfer energy directly to living tissue.

13:53Safe handling must therefore prevent radioactive material entering the body as well as control external exposure. Radioactive materials should be moved in suitable shielded containers, rather than carried directly by hand. During use, handle sources with tongs or remote equipment and keep them away from the body. Store them in labelled, shielded containers in a secure location. Avoid touching exposed material, eating nearby or allowing contamination to spread. The Extended explanation uses three principles: reduce exposure time, increase distance and use shielding.

14:27Less time near a source means fewer opportunities for energy to reach living tissue. Greater distance reduces the amount reaching a person from a small source. Suitable shielding absorbs radiation before it reaches the body. Shielding must match the radiation and the practical situation. Paper stops alpha particles, aluminium can stop beta particles, and thick lead or concrete reduces gamma intensity. Gloves alone are not suitable protection against penetrating gamma radiation.

14:55Safe procedures combine time, distance and shielding rather than depending on only one precaution.

Exam answers and recap

15:00An exam-style question gives a background of twenty counts per minute and asks you to calculate the half-life from this table. Subtracting twenty gives corrected rates of three hundred and twenty, one hundred and sixty, and eighty counts per minute. The corrected rate halves during each four-minute interval. Therefore, the half-life is four minutes. If asked to explain why beta radiation suits paper thickness control, give separate creditworthy points. Some beta particles pass through the paper and some are absorbed.

15:28A thicker sheet absorbs more, reducing the detected count rate. Alpha particles would be stopped completely, while gamma radiation would be absorbed too little. For an explanation of safe handling, connect every precaution to its purpose. Tongs increase the distance between the source and living tissue. Short handling time reduces exposure. Suitable shielding absorbs radiation, reducing the amount reaching the person. These are explanations, rather than a list of equipment without reasons.

15:56Radioactivity begins with spontaneous, random changes in unstable nuclei. Alpha removes two protons and two neutrons, beta converts a neutron into a proton, and gamma removes excess energy. Charge helps explain ionisation and field deflection, while penetration determines useful shielding and applications. Half-life describes repeated halving, with background subtracted before interpreting source readings. The same properties explain both the benefits and dangers of nuclear radiation. Strong ionisation can detect smoke, kill bacteria or damage living cells.

16:29Penetration determines where the energy can reach, and half-life determines how the source changes over time. Understanding those relationships lets you explain the physics instead of memorising isolated facts.