Antiparticles, pair production and annihilation

A sub‑atomic particle has a positive charge.

Which type of particle is it?

A anti‑proton

B down quark

C neutrino

D positron

Your answer

A 3D printer can manufacture small objects.

Some 3D printers use polylactic acid (PLA). PLA is supplied in the form of long filaments. The 3D printer melts the PLA and builds up the shape of the desired object in layers.

The electrical supply to the heater in the printer has an e.m.f., , of 12 V. The power of the heater is 40 W.

Calculate the resistance, R, of the heater.

The specific latent heat of fusion of PLA is 9.4 × 10⁴ J kg^–1 and its melting point is 160 °C. (b)

Define specific latent heat of fusion.

Calculate the maximum mass m of PLA that the heater could melt in one minute.

Explain why the printing process is slower in practice than your answer to (ii) suggests.

High-energy X-ray photons can destroy living cells. In radiotherapy, these photons are targeted at cancer cells.

The radiation dose is the amount of energy that a patient’s body absorbs from the high-energy X-ray photons.

Fig. 6.2 shows how this dose changes with depth below the surface of the skin.

The dose initially rises with depth because the high-energy X-ray photons produce electrons and positrons as they pass through the body. These electrons and positrons are quickly absorbed, increasing the dose.

Explain why high-energy X-ray photons produce electrons and positrons as they pass through the body.

*(ii) A 3D object called a bolus is used in radiotherapy for patients with skin cancer. A bolus targets the maximum radiation dose near the surface of the skin. So using a bolus makes the radiotherapy more effective.

A bolus can be made from PLA using a 3D printer. The bolus must fit the shape of the patient’s body exactly. This shape is found beforehand by giving the patient either a CAT scan or a PET scan.

• Explain how CAT scans and PET scans work.

• Discuss the advantages and disadvantages of having a scan to produce a bolus for radiotherapy.

Fig. 24 shows two horizontal metal plates in a vacuum.

The diagram is not drawn to scale. Electrons travelling horizontally enter the space between the charged plates and are deflected vertically.

The potential difference between the plates is 4000 V. The distance between the plates is 0.08 m. The initial speed of the electrons is 6.0 × 10⁷ m s^–1. The vertical deflection of the electrons at the far end of the plates is x.

The length of each plate is 0.12 m.

Show that the time t taken by the electron to travel this length is 2.0 × 10^–9 s.

Calculate the vertical deflection x of the electron.

The arrangement shown in Fig. 24 is now used to investigate positrons emitted from a radioactive source. The speed of the positrons is also 6.0 × 10⁷ m s^–1.

The initial path of the positrons is the same as that of the electrons in Fig. 24.

On Fig. 24, sketch the path of the positrons between the plates.

Beta-minus particles (electrons) emitted from a radioactive source have a range of speeds.

Describe and explain how a uniform magnetic field can be applied in the space between the charged plates to select beta-minus particles with a specific speed. No calculations are required.

X-ray photons interact with atoms.

The attenuation coefficient against lg(photon energy) graphs for simple scattering (S), photoelectric effect (PE), Compton effect (C) and pair production (PP) are shown below.

With the help of a calculation, explain the minimum photon energy shown on the graph for pair production.

Which statement is correct?

A Hadrons are made up of protons and neutrons.

B A positron and a proton are examples of leptons.

C The positron and the electron have the same mass.

D The weak nuclear force is responsible for alpha-decay.

Your answer

Stars produce energy by nuclear fusion. One particular fusion reaction between two protons (¹₁H) is shown below.

In this reaction 2.2 MeV of energy is released.

Only one of the particles shown in the reaction has binding energy. Determine the binding energy per nucleon of this particle. Explain your answer.

Explain why high temperatures are necessary for fusion reactions to occur in stars.

A gamma photon in a star can spontaneously create an electron-positron pair. Calculate the maximum wavelength of a gamma photon for this creation event.