Fundamentals

Question 1

Exposure X

Answer 1

related to the ability of photons to ionize air. Its unit röntgen (R)
is defined as charge of 2.58×10−4 C produced per kilogram of air.

Question 2

Kerma K

Answer 2

defined for indirectly ionizing radiations (photons and neutrons) as
the energy transferred to charged particles per unit mass of the absorber. Its SI
unit gray (Gy) is defined as 1 J of energy absorbed per kilogram of medium,
i.e., 1 Gy = 1 J · kg−1.

Question 3

Dose D

Answer 3

energy absorbed per unit mass of medium. Its SI unit
gray (Gy) is defined as 1 J of energy absorbed per kilogram of medium, i.e.,
1 Gy = 1 J/kg. The old unit of dose is rad where 1 Gy = 100 cGy = 100 rad =
100000 mrad.

Question 4

Equivalent dose H

Answer 4

dose multiplied by a radiation-weighting
factor wR. The SI unit of equivalent dose is sievert (Sv), the old unit is rem
where 1 Sv = 100 rem.

Question 5

Activity A

Answer 5

defined as the number of nuclear decays
per unit time. Its SI unit is becquerel (Bq) corresponding to one decay per
second or 1 Bq = 1 s−1.

Question 6

Thermionic emission (TE) and field emission (FE) are physical phenomena
of importance not only in theoretical physics but also in practical production
of x rays.

In point form compare thermionic emission (TE) from a metal with field
emission (FE) from a metal.

Answer 6

In both the thermionic emission (TE) and field emission (FE) electrons are
emitted from the surface of a metal. On a microscopic scale, however, the two effects
are different:
(1) In TE electrons surmount the potential barrier, while in FE electrons tunnel
through the potential barrier.
(2) In TE the temperature T of the metal increases the energy of electrons to allow
them to surmount the potential barrier; in FE a strong external electric field
E narrows the potential barrier to make the tunneling of electrons through the
barrier possible.
(3) In TE the work function remains constant with temperature but the kinetic
energy of electrons rises with metal temperature, allowing electrons to escape
from the metal. In FE the barrier thickness diminishes with an increasing applied
electric field, making it easier for electrons to tunnel through the barrier.
(4) The higher the temperature T of the metal, the stronger is the emission of
electrons in TE; the stronger is the external electric field E, the stronger is the
emission of electrons in FE.
(5) TE is of importance in the fields of electronics and communications in general.
In medical physics TE plays an important role in hot cathode x-ray tubes
(Coolidge tubes) and electron guns in linear accelerators. FE shows promise
in the development of cold cathode x-ray tubes.
(6) The form of the functional dependence of electron current density j on T in
TE is the same as that on E in FE.