Page 103 - 6848

A 1 MeV ("M" is the abbreviation for mega, a prefix that multiplies a basic unit by
1,000,000) gamma ray, with energy 2,000,000 times that of visible light, can pass completely
through the body, creating tens of thousands of ions as it does.
X-rays and gamma radiation have low ionization potential (low specific
ionization) and high penetrating power. (They can be weakened by lead or concrete
wall with big thickness)

 A final form of radiation of concern is neutron radiation. Neutrons,
along with protons, are one of the components of the atomic nucleus. Like protons,
they have a large mass; unlike protons, they have no electric charge, allowing them to
slip more easily between atoms. Like a Stealth fighter, high-energy neutrons can travel farther
into the body, past the protective outer layer of the skin, before delivering their energy and causing
ionization.
Several other types of high-energy particles are also ionizing radiation. Cosmic radiation
that penetrates the Earth's atmosphere from space consists mainly of protons, alpha particles, and
heavier atomic nuclei. Positrons, mesons, pions, and other exotic particles can also be ionizing
radiation.

13.2 Units of Measurement of Ionizing Radiation

 Radioactivity.

As its name implies, radioactivity is the act of emitting radiation
spontaneously.

This is done by an atomic nucleus that, for some reason, is unstable; it "wants" to give up
some energy in order to shift to a more stable configuration. During the first half of the twentieth
century, much of modern physics was devoted to exploring why this happens, with the result that
nuclear decay was fairly well understood by 1960. Too many neutrons in a nucleus lead it to emit a
negative beta particle, which changes one of the neutrons into a proton. Too many protons in a
nucleus lead it to emit a positron (positively charged electron), changing a proton into a neutron.
Too much energy leads a nucleus to emit a gamma ray, which discards great energy without
changing any of the particles in the nucleus. Too much mass leads a nucleus to emit an alpha
particle, discarding four heavy particles (two protons and two neutrons).

How is radioactivity measured?
Radioactivity is a physical, not a biological, phenomenon.
Simply stated, the radioactivity of a sample can be measured by counting
how many nuclei are spontaneously decaying N during the time dt.

dN
A .
dt
This can be done with instruments designed to detect the particular type of
radiation emitted with each "decay" or disintegration.
The actual number of disintegrations per second may be quite large. Scientists
have agreed upon common units to use as a form of shorthand (table 13.2). Thus, a
curie (abbreviated Ci and named after Pierre and Marie Curie, the discoverers of

4