Experimental Nuclear Physics

Course requirements for Module 2 - Experimental Nuclear Physics

• 2.1 Scattering Experiments (Questions)

• 2.2 Modelling Scattering (Simulation and Data Analysis activity)

• 2.3 Modelling Scattering (Questions)

• 2.4 Detecting Radiation (Questions)

• 2.5 Detecting Protons (Questions)

In Rutherford's scattering experiment, an alpha particle was scattered from a gold nucleus. Assuming that the only force is coulomb repulsion (between the two positive charges), we can approximate the radius (r) of the gold nucleus by calculating the point of closest approach of the alpha particle.  

If the alpha particle initially has a kinetic energy, Ek , this is converted to electric potential energy, Ep , at the point of closest approach.  

You can download and run the full Scattering Simulation below. This will allow you to view the scattering experiment for a range of different target shapes, as well as varying the ramp angle (and thus the energy of the ball bearings). 

Please note that this software is only suitable for Windows computers.  

• Download the .zip file.

• Unzip/extract the file contents.

• Double-click on the file titled 'Scattering_Game.blend1' within the 'game' folder and follow the instructions to install it. 

• Once installed, click 'Controls' to learn how to play the game, and then 'Play Game' to start.

  • Use the up and down arrows on your keyboard to change the ramp angle. Use A and D to move left and right, and S and W to zoom in and out.

  • To select the circular target, press z and to select the triangular target, press x.

  • Place the ball bearings on the ramp by pressing 2 and release them by pressing the space bar.  You should now be able to observe the scattering process. 

• Keep repeating this with different combinations - you will need to exit the game to reset the simulation.  

Gaseous ionisation chambers utilise the ionising effect of radiation to generate electrical signals. A charged particle passing through the detector will ionise the gas molecules - giving an atomic electron sufficient energy to escape the atom and leaving a positive ion and negative electron. A voltage applied between the anode and cathode (see the diagram below) creates an electric field, which causes the ions to drift to opposite sides. The collection of these ions on the electrodes will register a current, signalling the presence of a particle. The Geiger Muller counters you may already be familiar with are examples of gaseous ionisation detectors. Photons can also cause ionisation. However, this type of detector is much less sensitive to photons compared to solid state detectors.

There are different types of solid-state detector systems:

• Semiconductor Detectors utilise semiconductors such as silicon or germanium. Unlike a metal conductor, a semiconductor can only conduct if some of the electrons are given additional energy to allow them to move freely in the material. Charged particles or photons interact with the semiconductor to produce these free electrons, leaving a ‘hole’ behind. Typically, the number of electron-hole pairs is proportional to the energy deposited by the particle or photon. The electrons and holes travel to the electrodes under the influence of an electric field, and an electric pulse registers the passage of ionising radiation through the system. Semiconductor detectors provide better energy measurements than gaseous systems. This is because electrons can be freed at lower energies in semiconductors than in gas detectors. However, semiconductor detectors are more expensive.

• Scintillation counters use materials which emit photons (scintillate) in response to radiation. A charged particle going through a scintillator excites molecules along its path, which then de-excite by the emission of a photon. The emitted photon is then detected by a light sensor such as a photomultiplier tube or photodiode, which converts the light to an electronic signal for reading.

• Cherenkov detectors allow the detection of particles with speeds greater than that at which light can travel through the material. Although we tend to think of the speed of light as the ‘speed limit’ of the universe, the speed of light varies depending on the medium it is passing through. This is why we see refraction when light enters water or a glass block. Some particles are not slowed to the same extent in different media, so can travel faster than the speed of light in these conditions. Particles travelling through a medium at speeds larger than the speed of light within the medium emit light in a process similar to sonic booms with sound. By carefully choosing the medium with an appropriate refractive index, this system allows us to identify particle types.

Tracking detectors reveal the path or ‘track’ taken by a particle. If we also use a magnetic field, we can work out a particle’s momentum and charge. This is because a magnetic field perpendicular to the direction of the particle motion will exert a force, causing the particle to follow a curved path. The radius of the curve is directly proportional to the particle momentum, and the direction of the curve reveals the charge of the particle.

Other types of detectors, called calorimeters, are used to measure particle energies. These detectors rely on the interaction of particles with matter. The particles are completely stopped within the detector and all of their energy is absorbed. Different detector systems need to be employed to detect, identify, and measure the characteristics of different types of particles. Therefore, many modern particle detectors are composed of layers of sub-detectors, each designed to look for particular properties or specific types of particle. For example, the Atlas detector at CERN has six different detecting subsystems and, at 46m long, 25m high and 25m wide, it is the largest volume particle detector ever constructed. 

In the video below, nuclear physicists Dr Marina Petri (University of York), Dr Marc Labiche (STFC Daresbury Laboratory), and Dr Carl Unsworth (STFC Daresbury Laboratory)  explain why a tracking detector for protons was needed, and the process of designing and building this device.