Nuclear Physics LATEST EDITION 2024/25 GUARANTEED GRADE A+

Nuclear Physics LATEST EDITION 2024/25 GUARANTEED GRADE A+ Properties of α radiation 2 Alpha particles can only be emitted by nuclei with an atomic number greater than 82. Typically, alpha particles cannot penetrate through a sheet of paper or a few mm of air. Rutherford Scattering In the early 20th century Ernest Rutherford conducted an experiment which completely changed our model of the atom. The experiment Rutherford conducted an experiment where he fired alpha particles at a very thin piece of gold leaf. The particles mostly passed through the gold leaf, but some were deflected at large angles. This was not expected and disproved the 'plum pudding model'. Explanation of results The strong deflections were caused by electrostatic repulsion between the positive nucleus and the positive alpha particles. Most alpha particles passed through as the atom is mostly free space. This new theory became our current model of the atom, a positive nucleus with negative electrons in orbit. Why did most alpha particles pass through the gold leaf? Atoms are mainly empty space Gold has little mass Alpha particles have no mass There were holes in the leaf Atoms are mainly empty space Democritus In the 5th century BC, Democritus suggested that matter was made of lumps called atomos. All atomos were believed to be identical. The atomo was considered the smallest possible unit of matter. Dalton In the 1800s, Dalton agreed with Democritus. However, he believed that different types of atom corresponded to different elements. Thomson Thomson discovered that electrons could be removed from atoms. This showed that atoms were not the smallest units of matter. Thomson believed that atoms were clouds of positive charge with negative electrons suspended inside.This was called the plum-pudding model. Rutherford Rutherford used his famous scattering experiment to show that atoms consist of a positively charged nucleus with orbiting negatively charged electrons. This is the current day model of the atom. Properties of α radiation Alpha particles are: Two protons and two neutrons bound together (the equivalent of a helium nucleus) Weakly penetrating Easily absorbed Positively charged Properties of β radiation Beta particles are: Electrons (or positrons) that are emitted from an unstable nucleus Moderately penetrative Either positively or negatively charged Beta plus particles emitted by proton rich nuclei Beta minus particles emitted by neutron rich nuclei. Applications of beta particles Typically, beta particles cannot penetrate through about 5 mm of aluminium or about 30 cm of air. A beta source is used for gauging the thickness of aluminium sheeting or foil. A beta-plus emitter is used in medical PET scanning. Applications of α radiation An alpha source is used in fire alarms. Alpha particles cannot penetrate through smoke. This is detected and sets off the alarm. Beta particles are: 1 Moderately penetrative 2 Either positively or negatively charged 3 Positrons or electrons Radioactivity Safety All institutions that hold radioactive sources need a permit to do so. The institution will have to follow some local rules about storage and use of sources. Protective measures should include: Keeping the source at arm's length Tongs should be used when moving the source. Protective clothing Wearing goggles and gloves Reducing exposure time Put the source away when it's not in use so as to reduce exposure time. Pointing sources away Sources should be pointed away from people including yourself. Protective box Keep the source in a lead-lined box. Typically, beta particles cannot penetrate through about 5 mm of aluminium or about 30 cm of _____ air Properties of γ radiation Gamma radiation: Does not change the proton or nucleon number of a nucleus. Makes the nucleus more stable by its emission Is highly penetrative Can be absorbed by several centimetres of lead, many metres of air and can travel through a vacuum indefinitely. Properties of γ radiation 2 Gamma radiation is part of the electromagnetic spectrum. Gamma radiation follows an inverse square law:intensity at distance x from the source =constant /x^2 Applications for gamma radiation Gamma radiation is used extensively in medical imaging, curing cancer by destroying tumourous cells and for sterilising medical equipment. It can be used to irradiate food to stop food from going bad. Investigating the Inverse Square Law Gamma radiation follows an inverse square law. We can perform an experiment to show that this is true. 1) Find background radiation Do a background count with a Geiger-Muller (GM) tube and counter without a source present. Repeat this measurement and find an average background count per minute. 2) Experimental setup and method Position the source in a holder. Position the GM tube at different distances from the source. Measure the count rate at each distance several times and find an average for each distance. CONTINUED...