30.1 X-rays and radioactivity Nature of X-rays X-rays are a kind of electromagnetic wave of high frequency (i.e. short wavelength). radio wave micro- wave infrared radiation visible light ultraviolet radiation X-ray gamma ray increasing frequency Electromagnetic spectrum

X-rays can be produced by rapid deceleration of high-speed charged particles, for example, hitting a heavy metal target with high-speed electrons. X-ray tube

Low-frequency X-rays can pass through flesh but not through bones. Depending on the frequency of the X-rays as well as the medium through which they transmit, they lose energy to different extent in transmission. flesh bone low-frequency X-rays Low-frequency X-rays can pass through flesh but not through bones. X-rays are also a kind of ionizing radiation. Ionization is the process in which one or more electrons are removed from an atom, which then becomes an ion.

Applications of X-rays Medical use Radiographic imaging Computed tomography (CT) scan An X-ray image of lungs A CT scan image of a patient’s head

X-ray inspection in airports Security use X-ray inspection in airports An X-ray image of luggage An X-ray image of a passenger in an airport Industrial use Detecting defective products at the point of production

30.3 Radiation safety Radiation dose The amount of radiation absorbed by a unit mass of a material is the dose. Its unit is the gray (Gy) or J kg−1. Equivalent dose describes the biological effect on a tissue or organ due to different types of radiation. Effective dose is the weighted equivalent dose that describes the biological effect on the whole human body. The unit of equivalent dose and effective dose is the sievert (Sv) or millisievert (mSv).

typical effective dose Potential hazards artificial source natural source typical effective dose source cosmic ray (depending on altitude) 0.25–0.35 mSv per year food and water 0.4 mSv per year radon in average household 2 mSv per year X-ray diagnosis 0.4 mSv each time 1000 mile flight trip 0.01 mSv each time cigarette smoking (1 pack per day) 15–20 mSv per year Effective doses from various sources Example 30.5

Safety precautions In radiation fields Workers have to follow the ‘ALARA (As Low As Reasonable Achievable) principle’, which means to reduce exposure to radiation to the lowest possible level. Workers wearing protective clothing Handling highly radioactive materials with robot arms

In school laboratories Sources should be handled with special forceps. Never use your hands to touch the radioactive source directly. Sources should be kept away from the body, especially the eyes, and should not be pointed to anybody. Sources should be stored in a suitable lead container and kept locked when not in use.

Warning signs should be labelled on both the containers and the storage room. Wash your hands thoroughly after conducting experiments. Report to the Radiation Board of the Department of Health immediately if the source is lost or is damaged.

In medicine During a radioactive medical treatment, both the patient and the radiotherapist should be protected from over exposure to radiation. The diagnosis room for radiotherapy is a controlled area. The radiotherapist works behind a lead shield when the patient is undergoing treatment.