Section 3 Medical Literature 第三部分 医学文献
Part 1.
In addition to x-ray therapy,radioactive substances called radioisotopes are used in several ways to treat cancer.The use of radioactive substances to treat cancer or as tracer substances in an organ(called tagged1) substances)has become a specialized field within radiology known as nuclear medicine.Radioisotopes have two significant advantages over conventional x-ray therapy.First，the powerful emission produced by a radioisotope is without any electrical equipment.Secondly，skin and bone damage—common in x-ray therapy—is significantly reduced by using radioisotopes.Here are some forms of treatment employing radioisotopes.
Teletherapy.The radioactive substance is kept in a protective container at a distance(6 to 7 feet)from the patient.Gamma rays are emitted from the radioisotopes,such as cobalt-602) or cesium-1373)，and absorbed by the patient.
Radioisotope molds.The radioactive substance is either sealed in a small container and placed directly onto the affected area or it is dispersed onto some material，such as plastic or paper，that is attached directly to the skin.
Intracavitary isotope therapy.The radioisotope is placed directly into a body cavity or hollow organ.For example，radioactive colloidal gold4) or sodium5) is placed in a balloon inside the bladder.Encapsulated radioactive substances are placed directly into certain hollow areas of the body—for example，the uterus.
Interstitial isotope therapy.Radioisotopes in the form of needles called seeds are implanted directly into the tumor.
Radioisotope injections.The effectiveness of radioactive solutions injected into the body is based on their affinity6) for certain body tissues.Radioactive iodine,for instance,is effective in treating malignant thyroid tissue because it concentrates in thyroid tissue.Similarly，radioactive phosphorus7) has an affinity for bone tissue.The tissue in which a radioactive substance accumulates is called a target.
注释：
1）tag 标记
2）cobalt-60 钴-60(https://www.daowen.com)
3）cesium-137 铯-137
4）radioactive colloidal gold 放射性胶质金
5）sodium 钠
6）affinity 亲和力
7）phosphorus 磷
Part 2.
Electromagnetic radiation The complete range of electromagnetic radiation is made up of gamma rays,X-rays,ultra-violet rays,ordinary visible light,infrared radiation，and radio waves.All these are given out by the Sun，but some of them do not reach us，as the atmosphere behaves like a filter.It lets through visible light and certain radio waves；most of the other radiation is absorbed.This is fortunate for us,because gamma rays,X-rays,and ultra-violet rays can harm living things.Enough ultra-violet rays get through for us to detect themthey are responsible for our getting sunburn when we expose ourselves too rapidly in sunbathing.
A well-known characteristic of light is that it travels in straight lines.The other electromagnetic radiation behave in a similar way.For this reason scientists thought Marconi was wasting his time when he attempted to send radio waves from Cornwall to Newfoundland in 1902.It seemed obvious that the curvature of the Earth across the Atlantic would prevent this.However，the experiment was successful，and once again it had been shown that experiments should always be made to check predictions from theory.
A new theory was soon developed to explain the new facts，this was that a layer existed in the upper atmosphere which could reflect radio waves.In this way they would be bounced round the world.This was called the Heaviside layer，after one of the scientists who predicted it；but as later work showed that there were several layers at different heights，they are now referred to by letters，and the general region in which they are present is called the ionosphere.
The D-layer occurs at a height of fifty to sixty miles and disappears during the night；the E-layer occurs at sixty to seventy miles and is weakened but still present at night.The F-layer extends from ninety miles upwards，and is present as two layers during the day.The layers are able to conduct electricity because the particles of atmospheric gases are ionized—that is to say,split into ions and electrons.
An atom of any substance consists of a positively-charged central body(the nucleus)surrounded negatively-charged electrons，like planets around sun.Each electron bears a single negative charge; and usually the nucleus carries a number of positive charges equal to the number of electrons.Therefore the whole atom is neither positive nor negative.One or more electrons are removed，the remainder of the atom must be positively charged.It is called a positive ion.Sometimes it is possible to add extra electrons to the atoms，and then we get negative ions.
Electric currents flowing in wires are streams of electrons running through the wire.Also flow in a television tube，where they are made to hit the screen，causing a flash of light.Ions can also act as a current of electricity in liquids or gases，and thus the presence of electrons and ions in the upper atmosphere makes it electricity conducing.In fact，if there is a wind in the ionosphere，it will be an electric current.