Radiation Physics Pearls - Roach 1

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Radiation Physics

Gamma rays are produced ______ and X-rays are produced _____

Gamma rays are produced intranuclearly (e.g., radioactive decay) and X-rays are produced extranuclearly (e.g., linear accelerator).

Proton mass >,<,=,~ Neutron mass ?

Proton mass ~ Neutron mass ~1.01 atomic mass units (amu);

mass-energy equivalence is described by

mass-energy equivalence is described by Einstein's famous E = mc2; therefore, 1 amu = 931.5 MeV (electron volt) defined as the kinetic energy acquired by passing an electron though a potential difference of 1 V.

Arrangement of electrons is in orbits or shells denoted by _____. Maximum number of electrons per orbit is _____

Arrangement of electrons is in orbits or shells denoted by K (innermost), L, M, N, O, etc. Maximum number of electrons per orbit is 2n2 (where n depends on shell, K = 1, L = 2, etc.).

Four fundamental forces of nature in order of decreasing strength: ____

Four fundamental forces of nature in order of decreasing strength: strong nuclear, electromagnetic, weak nuclear, gravity

The binding energy of electrons refers to the

The binding energy of electrons refers to the magnitude of force (in Coulombs) between the electrons and nucleus;

high Z atoms have >,<,,= binding energies because of >,<,= nuclear charge;

high Z atoms have greater binding energies because of greater nuclear charge;

if inner orbital electrons are ejected from the atom, they will be filled by lower or higher orbital electrons resulting in _____ ___ ____

if inner orbital electrons are ejected from the atom, they will be filled by higher orbital electrons resulting in characteristic X-ray production

Nuclei are most stable at certain numbers of nucleons (neutrons + protons):______

Also nuclei with odd numbers of protons and neutrons are >,<,= stable than those with even
numbers of both.

Nuclei are most stable at certain numbers of nucleons (neutrons + protons): 2, 8, 20, 82, 126.

Also nuclei with odd numbers of protons and neutrons are less stable than those with even numbers of both.

The rate of nuclear decay (or radioactivity) is described by _____, where N is activity at time (t) and N0 is initial activity and l is the rate decay constant;

The rate of nuclear decay (or radioactivity) is described by N = Noe^(−ht), where N is activity at time (t) and No is initial activity and h is the rate decay constant;

Activity can be described in curies (Ci) where 1 Ci = ______ dps (disintegrations/sec);
1 dps = 1 Becquerel (Bq) =

activity can be described in curies (Ci) where 1 Ci = 3.7 × 1010 dps (disintegrations/sec);
1 dps = 1 Becquerel (Bq) = 2.7 × 10−11 Ci

The half-life (T1/2) of a radioisotope has been reached when N = ____ * (No)

When N = 0.5(N0), the half-life (T1/2) of a radioisotope has been reached;

The half-life (T1/2) of a radioisotope can also be described as T1/2 =_____

(T1/2) = 0.693/h;

the mean life (Tave) or average lifetime for decay of a radioactive nucleus can be described as

Tave = 1/h = 1.44*(T1/2).

Radioactive equilibrium refers to the

Radioactive equilibrium refers to the ratio between the activity of the parent isotope and its daughter product

In ______ equilibrium, the T1/2 of the parent is not too much greater than T1/2 of the daughter.

In transient equilibrium, the T1/2 of the parent is not too much greater than T1/2 of the daughter.

In _____ equilibrium, the half-life of the parent isotope is much longer than that of the daughter.

In secular equilibrium, the half-life of the parent isotope is
much longer than that of the daughter.

Alpha decay

Q = energy released

Positron decay (b plus decay)

v = n e u t r i n o ; Q = e n e r g y released;
produces positrons (useful in nuclear medicine)

Negatron decay (b minus decay)

~n = antineutrino; Q = energy released;
common in reactorproduced isotopes (e.g., 60Co)

Electron capture

An orbital electron (usually from K shell) is captured
by nuclear proton which is converted to neutron;
competitive with positron decay in nuclei with neutron
deficiencies

Internal conversion

A gamma ray is ejected from the nucleus, and in turn,
ejects an orbital electron;

the gamma ray is completely absorbed; the orbital vacancy
is filled by an outer shell electron resulting in emission of a characteristic X-ray

A gamma ray is ejected from the nucleus, and in turn, ejects an orbital electron;
the gamma ray is completely absorbed; the orbital vacancy is filled by an outer shell electron resulting in emission of a characteristic X-ray

Internal conversion

An orbital electron (usually from K shell) is captured
by nuclear proton which is converted to neutron;

Electron capture

competitive with positron decay in nuclei with neutron deficiencies

Electron Capture

common in reactorproduced isotopes (e.g., 60Co)

Negatron decay
(b minus decay)

produces positrons
(useful in nuclear medicine)

positron decay

In linear accelerators (linacs), ____ are accelerated through an ____ field and are rapidly _____ in a target material such as _____. This results in the production of X-rays of varying energies. The basic unit of X-rays are ____.

In linear accelerators (linacs), electrons are accelerated through an electric field and are rapidly decelerated in a target material such as tungsten. This results in the production of X-rays of varying energies. The basic unit of X-rays are photons.

X-ray production can be achieved by two major mechanisms:

X-ray production can be achieved by two major mechanisms:

Bremmstrahlung radiation, an accelerated electron changes direction when it comes into the proximity of a positively charged nucleus, resulting in photon production.

Characteristic X-rays are produced when an accelerated electron knocks an inner orbital electron out of its shell. This causes an outer shell electron to fill in the vacancy which subsequently results in photon production. The energy of this photon is the difference in binding energies of the two electrons.

An accelerated electron changes direction when it comes into the proximity of a positively charged nucleus, resulting in photon production. = ?

Bremmstrahlung radiation,

______ are produced when an accelerated electron knocks an ____orbital electron out of its shell. This causes an ____ shell electron to fill in the vacancy which subsequently results in photon production.

Characteristic X-rays,
Inner
Outer

What is the energy of the photon produced in the process of generating a charactertistic X-ray

The energy of this photon is the difference in binding energies of the two electrons (accelerated e- and inner orbital e-)

What two types of e- have to interact to produce a Characteristic X-ray

accelerated e- and inner orbital e-

Photon beams are attenuated as they pass through matter and the degree of attenuation depends on _______ and ____

Photon beams are attenuated as they pass through matter
and the degree of attenuation depends on both the thickness
(x) and the linear attenuation coefficient (μ) of the material.

What is the formula relating photon attenuation to material type and thickness

I(x) = Io e^(-μx).
Io represents the intensity of the beam prior to attenuation,
μ has units of (distance)−1 and it represents the fraction of incoming photons that are removed from the beam per unit thickness of material
x = thickness of materal
u = linear attentuation coefficient

Common photon energies and attenuation properties
for : Co-60
Energy
~Tissue attenuation/cm (%)
~Dmax (cm)

(1.25 MV)
5
0.5

Common photon energies and attenuation properties
for 6MV photon?
~Tissue attenuation/cm (%)
~Dmax (cm)

3.5
1.5

Common photon energies and attenuation properties
~Tissue attenuation/cm (%)
~Dmax (cm)

2.4
3.0

The mass attenuation coefficient (um) is equal to ______

The mass attenuation coefficient (um) is equal to u/p where p is the density of the material (in gm/cm3).

Unlike the linear attenuation coefficient, the mass attenuation coefficient does/does not vary much for different materials for photons in the therapeutic range.

Unlike the linear attenuation coefficient, the mass attenuation coefficient does not vary much for different materials for photons in the therapeutic range.

The half-value layer [HVL] (e.g., the thickness of a given material required to attenuate the beam intensity to one-half) can be expressed as:

HVL in relation to the mass attenuation coefficient

If all photons are of the same energy (monoenergetic), the first HVL is >,<,= to subsequent HVLs.

However, for polyenergetic photons, the first HVL is >,<,= than subsequent HVLs because of ___ ____.

If all photons are of the same energy (monoenergetic), the first HVL is identical to subsequent HVLs.

However, for polyenergetic photons, the first HVL is smaller than subsequent HVLs because of beam hardening. In other words, more material is required to remove the remaining higher energy photons.

Summary of major photon interactions
for: Compton scattering
-Brief description
-Prevalent at which energies in tissue?
-Dependence of mass attenuation coefficient
on atomic number

A photon hits an outer orbital electron causing it to be ejected from an atom; the photon is itself scattered

30 keV < E < 25 MeV (Linacs)

Nearly independent of Z (proportional to electron density
and provides poor contrast)

Summary of major photon interactions
for: Pair production
-Brief description
-Prevalent at which energies in tissue?
-Dependence of mass attenuation coefficient
on atomic number

-A photon hits the nucleus and produces an electron and positron
-E > 5 MeV (present) E > 25 MeV (dominant)
-Z

Summary of major photon interactions
for: Photoelectric effect
-Brief description
-Prevalent at which energies in tissue?
-Dependence of mass attenuation coefficient
on atomic number

Accelerated electron knocks inner orbital electron out of its shell; this leads to outer orbital electron filling in vacancy and
production of characteristic X-ray

E < 30 keV (diagnostic radiology)

Z3 (attenuation is variable based on Z of material; this results in good contrast between air, tissue, and bone)

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