Nuclear Physics Flashcards

Question

What are precautions for the radiographer during radiation therapy

Answer 1

The radiographer should handle the source remotely with tongs or a machine The radiographer should be protected by a screen The radiographer should be a long way from the source while the dose is given The source should be immediately stored in its lead case once the dose is given

Answer 2

Initially, the activity is very high, so only a small sample needed The shorter the half-life, the faster the isotope decays Isotopes with a shorter half-life pose a much lower risk to the patient The medical test doesn‘t last long so a half-life of a few hours is enough

Answer 3

Gamma radiation

Answer 4

It is the most penetrating out of all the types of radiation It is penetrating enough to irradiate all sides of the instruments Instruments can be sterilised without removing the packaging

Answer 5

In order for a substance to become radioactive, the nuclei have to be affected Ionising radiation only affects the outer electrons and not the nucleus The radioactive material is kept securely sealed away from the packaged equipment so there is no chance of contamination

Answer 6

to verify the inverse square law for gamma radiation of a known gamma-emitting source

Answer 7

Independent variable = the distance between the source and detector, x (m) Dependent variable = the count rate / activity of the source, C Control variables The time interval of each measurement The same thickness of aluminium foil The same gamma source

Answer 8

1. Measure the background radiation using a Geiger Muller tube without the gamma source in the room, take several readings and find an average 2. Next, put the gamma source at a set starting distance (e.g. 5 cm) from the GM tube and measure the number of counts in 60 seconds 3. Record 3 measurements for each distance and take an average 4. Repeat this for several distances going up in 5 cm intervals

Answer 9

According to the inverse square law, the intensity, I, of the γ radiation from a point source depends on the distance, x, from the source 1. Square each of the distances and subtract the background radiation from each count rate reading 2. Plot a graph of the corrected count rate per minute against 1/x2 3. If it is a straight line graph through the origin, this shows they are directly proportional, and the inverse square relationship is confirmed

Answer 10

The Geiger counter may suffer from an issue called “dead time” - This is when multiple counts happen simultaneously within ~100 μs and the counter only registers one -This is a more common problem in older detectors, so using a more modern Geiger counter should reduce this problem The source may not be a pure gamma emitter -To prevent any alpha or beta radiation being measured, the Geiger-Muller tube should be shielded with a sheet of 2–3 mm aluminium

Answer 11

Radioactive decay is random, so repeat readings are vital in this experiment Measure the count over the longest time span possible -A larger count helps reduce the statistical percentage uncertainty inherent in smaller readings -This is because the percentage error is proportional to the inverse-square root of the count

Answer 12

For the gamma source: - Reduce the exposure time by keeping it in a lead-lined box when not in use - Handle with long tongs - Do not point the source at anyone and keep a large distance (as activity reduces by an inverse square law) Safety clothing such as a lab coat, gloves and goggles must be worn

Answer 13

((Worst gradient — best gradient) / (best gradient)) x 100%

Answer 14

The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta or gamma particle

Answer 15

There is an equal probability of any nucleus decaying It cannot be known which particular nucleus will decay next It cannot be known at what time a particular nucleus will decay The rate of decay is unaffected by the surrounding conditions It is only possible to estimate the proportion of nuclei decaying in a given time period

Answer 16

Using a Geiger-muller (GM) tube When a GM tube is placed near a radioactive source, the counts are found to be irregular and cannot be predicted Each count represents a decay of an unstable nucleus These fluctuations in count rate on the GM tube provide evidence for the randomness of radioactive decay

Answer 17

the average number of nuclei that are expected to decay per unit time

Answer 18

The probability that an individual nucleus will decay per unit of time

Answer 19

N = N0 e^–λt N0 = the initial number of undecayed nuclei (when t = 0) N = number of undecayed nuclei at a certain time t λ = decay constant (s-1) t = time interval (s)

Answer 20

A = A0 e–λt

Answer 21

C = C0 e–λt

Answer 22

The number of atoms in one mole of a substance; equal to 6.02 × 1023 mol-1

Answer 23

The time taken for the initial number of nuclei to halve for a particular isotope

Answer 24

The difference between an atom's mass and the sum of the masses of its protons and neutrons

Answer 25

Δm = Zmp + (A – Z)mn – mtotal Where: Z = proton number A = nucleon number mp = mass of a proton (kg) mn = mass of a neutron (kg) mtotal = measured mass of the nucleus (kg)

Answer 26

Since nuclei are made up of neutrons and protons, there are forces of repulsion between the positive protons

Answer 27

The amount of energy required to separate a nucleus into its constituent protons and neutrons

Answer 28

It is less than the sum of the energies of its constituent nucleons

Answer 29

An exothermic reaction

Answer 30

1.661 x 10^-27

Answer 31

The mass of exactly one-twelfth of an atom of carbon-12

Answer 32

The fusing together of two small nuclei to produce a larger nucleus

Answer 33

The splitting of a large atomic nucleus into smaller nuclei

Answer 34

Fission must first be induced by firing neutrons at a nucleus When the nucleus is struck by a neutron, it splits into two, or more, daughter nuclei During fission, neutrons are ejected from the nucleus, which in turn, can collide with other nuclei which triggers a cascade effect This leads to a chain reaction which lasts until all of the material has undergone fission, or the reaction is halted by a moderator

Answer 35

Low mass nuclei (such as hydrogen and helium) can undergo fusion and release energy When two protons fuse, the element deuterium is produced In the centre of stars, the deuterium combines with a tritium nucleus to form a helium nucleus, plus the release of energy, which provides fuel for the star to continue burning

Answer 36

Binding energy = Binding Energy per Nucleon × Mass Number

Answer 37

1 MeV = 1.60 x 10^-13

Answer 38

The binding energy of a nucleus divided by the number of nucleons in the nucleus

Answer 39

1. In fusion, the mass of the nucleus that is created is slightly less than the total mass of the original nuclei - The mass defect is equal to the binding energy that is released since the nucleus that is formed is more stable 2. Fission occurs at high values of A because: - Repulsive electrostatic forces between protons begin to dominate, and these forces tend to break apart the nucleus rather than hold it together 3. In fission, an unstable nucleus is converted into more stable nuclei with a smaller total mass - This difference in mass, the mass defect, is equal to the binding energy that is released

Answer 40

1. Fusion releases much more energy per kg than fission 2. The energy released is the difference in binding energy caused by the difference in mass between the reactant and products - Hence, the greater the increase in binding energy, the greater the energy released 3. At small values of A (fusion region), the gradient is much steeper compared to the gradient at large values of A (fission region) 4. This corresponds to a larger binding energy per nucleon being released

Answer 41

Energy released = Binding energy after – Binding energy before

Answer 42

When a stable nucleus splits into small nuclei from the bombardment of a slow-moving neutron

Answer 43

Thermal neutrons - low energy and speed, so can induce fission - important as neutrons with too much energy will rebound away from uranium-235 nucleus and fission will not take place.

Answer 44

The minimum mass of fuel required to maintain a steady chain reaction

Answer 45

A single fission reaction follows the last. Using less than the critical mass (subcritical mass) would lead the reaction to eventually stop Using more than the critical mass (supercritical mass) would lead to a runaway reaction and eventually an explosion

Answer 46

To slow down neutrons The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’

Answer 47

To absorb neutrons Lowering the rods further decreases the rate of fission, as more neutrons are absorbed Raising the rods increases the rate of fission, as fewer neutrons are absorbed

Answer 48

To remove heat released by the fission reactions The coolant carries the heat to an external boiler to produce steam This steam then goes on to power electricity-generating turbines