01 radiation units
TRANSCRIPT
-
8/9/2019 01 Radiation Units
1/41
1
-
8/9/2019 01 Radiation Units
2/41
2
-
8/9/2019 01 Radiation Units
3/41
Radioactive decay is the process by which unstable
atoms transform themselves into new chemical
elements
Introduction
-
8/9/2019 01 Radiation Units
4/41
1 Bq = 1 disintegration per second
Activity
The amount of a radionuclide present
SI unit is the becquerel (Bq)
-
8/9/2019 01 Radiation Units
5/41
Multiples & Prefixes (Activity)
Multiple Prefix Abbreviation1 ------- Bq
1 x 106 Mega (M) MBq
1 x 109 Giga (G) GBq
1 x 1012 Tera (T) TBq
1 x 1015 Peta (P) PBq
-
8/9/2019 01 Radiation Units
6/41
UnitsCurie (Ci) = 3.7 x 1010 dps
Becquerel (Bq) = 1 dps (SI)
1 Ci = 3.7 x 1010 Bq
-
8/9/2019 01 Radiation Units
7/41
The Decay Constant is denoted by
NOTE: Units on are
Typically or sec-1 or “per second”
Decay Constant
1
time
1sec
-
8/9/2019 01 Radiation Units
8/41
A = N
where “A = activity” has units of
disintegrations per second(dps or Bq)
Activity
-
8/9/2019 01 Radiation Units
9/41
The half-life is the time required for an amount of any
radionuclide to decay to one-half of its initial value.
The relationship between half-life and decay constantis:
Half-Life and Decay Constant
T½ =0.693
-
8/9/2019 01 Radiation Units
10/41
Half-Life
-
8/9/2019 01 Radiation Units
11/41
Radioactive Decay
The amount of activity decayed away after“n” half -lives is given by
A
Ao
1 -
represents the fraction of the initial
activity that still exists at time t
A
Ao
-
8/9/2019 01 Radiation Units
12/41
Half-Life
Radionuclide Half-Life
Phosphorus-32 14.3 days
Iridium-192 74 days
Cobalt-60 5.25 years
Caesium-137 30 years
Carbon-14 5760 years
Uranium-238 4.5 x 109 years
-
8/9/2019 01 Radiation Units
13/41
Exercise 1
A criticality accident occurs in a Uranium processing facility.
1019 fissions occur over a 17 hour period. Given that the fissionyield for 131I is 0.03 and its half-life is 8 days, calculate the 131I
activity at the end of the accident. Neglect 131I decay during the
accident.
Activity = N = x
x ( 1019 x 0.03) = 3 x 1011 Bq 131I
0.693
8 days
1
86,400 sec day-1
3 x 1011 Bq
3.7 x 1010 Bq/Ci= 8.1 Ci 131I
-
8/9/2019 01 Radiation Units
14/41
Radioactive Decay Equation
N(t) = No e-
t
= -
N(t)dNdt
Expressing the equation in terms of activity:
N(t) = No e- t
A(t) = Ao e- t
where A(t) = activity at any time t
and Ao = the initial activity at time t = 0
or
Integrated
-
8/9/2019 01 Radiation Units
15/41
Mean Life and Half-Life
However, when t = T½, the activity decreases to ½ of the
original value:
A = Ao e- t
or
A
Ao = e- t
A
Ao =
½Ao
Ao = ½
½ = e - T½
Take the natural logarithm of both sides
ln (½) = -T½ ln (½) = ln (e ) - T½
-
8/9/2019 01 Radiation Units
16/41
1=
ln (½)
-T½ Regrouping terms yields
But ln (½) = - ln (2) so:
1=
- ln (2)
-T½
ln (2)
T½ =
but ln(2) = 0.693
1=
ln (2)
T½
Mean Life
= 1.44 T½ = Tm 1 =
0.693T½
where Tm , mean life,,
Mean Life and Half-Life
-
8/9/2019 01 Radiation Units
17/41
A radionuclide has a half life of 10 days. What is the
mean life?
Exercise 2
Mean Life = 1.44 T1/2
= 1.44 x 10 days
= 14.4 days
-
8/9/2019 01 Radiation Units
18/41
M = molecular weight of sample
Av= Avogadro's number(= 6.02 x 1023 nuclei/mole)
= r =ioisotope decay constant ( = ln 2 /half-life)
18
The specific activity of a radioactive source is defined as the
activity per unit mass of the radioisotope sample.
Activity can be calculated from
-
8/9/2019 01 Radiation Units
19/41
19
Calculate the specific activity of pure tritium (3H) with a half-
life of 12.26 years.
Substitute T1/2 =12.26 years and M=3 grams/mole to get the
specific activity in disintegrations/(gram – year).
Exercise 3
-
8/9/2019 01 Radiation Units
20/41
-
8/9/2019 01 Radiation Units
21/41
ENERGY
• It defined as the kinetic energy gained by an electron
which its acceleration through a potential differenceof 1 volt.
•
The unit for energy is the electron volt or eV
• The electron volt is a convenient unit because the
energy gained from an electric field can be easily
obtained by multiplying the potential difference by
the number of electronic charges carried by the
particle.
21
-
8/9/2019 01 Radiation Units
22/41
• For example, an alpha particle that carries an
electronic charge of +2 will gain an energy of 2 ke Vwhen accelerated by a potential difference of 1000
volts.
• SI unit of energy
1eV=1.602 x 10-19 J
1fJ(=10-15 J) = 6.241 x 103 eV
femtojoule (fJ)
22
-
8/9/2019 01 Radiation Units
23/41
• The energy of an X-or gamma-ray photon is related to
the radiation frequency by
where h = Planck's constant
(6.626 x 10-34
J · s, or 4.135 x 10-15
eV · s)v = frequency
The wavelength λ . is related to the photon energy by
where λ is in meters and E is in e V.23
-
8/9/2019 01 Radiation Units
24/41
24
What is the lowest wavelength limit of the X-rays emitted by a
tube operating at a potential of 195 kV?
Since an x-ray must essentially be created by the de-excitation
of a single electron, the maximum energy of an x-ray emitted in
a tube operating at a potential of 195 kV must be 195 keV.
E=hv E=hc/
Exercise 4
-
8/9/2019 01 Radiation Units
25/41
RADIATION UNITS AND DOSEQUANTITIES
IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources
-
8/9/2019 01 Radiation Units
26/41
Exposure : X
•Exposure is a dosimetric quantity for ionizingelectromagnetic radiation, based on the ability
of the radiation to produce ionization in air .
• This quantity is only defined for
electromagnetic radiation producing
interactions in air.
Diagnostic Radiology Part II : RadiationPhysics
26
-
8/9/2019 01 Radiation Units
27/41
•Exposure is the absolute value of the total chargeof the ions of one sign produced in air when all
the electrons liberated by photons per unit mass
of air are completely stopped in air.
Exposure : X
Diagnostic Radiology Part II : RadiationPhysics
27
X = dQ/dm
-
8/9/2019 01 Radiation Units
28/41
Exposure : X
•The SI unit of exposure is Coulomb per kilogram[Ckg-1]
• The former special unit of exposure was
Roentgen [R]
• 1 R = 2.58 x 10-4 Ckg-1
• 1 Ckg-1 = 3876 R
Diagnostic Radiology Part II : RadiationPhysics
28
/
-
8/9/2019 01 Radiation Units
29/41
Exposure rate: X/t Exposure rate (and later, dose rate) is the exposure
produced per unit of time.
The SI unit of exposure rate is the [C/kg] per second
or (in old units) [R/s].
In radiation protection it is common to indicate these
rate values “per hour” (e.g. R/h).
Diagnostic Radiology Part II : RadiationPhysics
29
Ab b d d D
-
8/9/2019 01 Radiation Units
30/41
Absorbed dose, D
• The absorbed dose D, is the energy absorbed per unit
mass.• This quantity is defined for all ionizing radiation (not
only for electromagnetic radiation, as in the case of
the “exposure”), and for any material.
• D = dE/dm. The SI unit of D is the Gray [Gy].
• 1 Gy = J/kg.
• The former unit was the “rad”. 1 Gy = 100 rad.
Diagnostic Radiology Part II : RadiationPhysics
30
-
8/9/2019 01 Radiation Units
31/41
Relation between absorbed dose and exposure
•
It is possible to calculate the absorbed dose in amaterial if the exposure is known
• D [Gy]. = f . X [Ckg-1]
–
f = conversion coefficient depending on medium
• The absorbed energy in 1 gram of air exposed
to 1 [Ckg-1] of X-rays is 0.869 [Gy]
f (air) = 0.869
Diagnostic Radiology Part II : RadiationPhysics
31
-
8/9/2019 01 Radiation Units
32/41
Example of conversion coefficient: f
f values (([[GGyy]] / [[CCk k gg--11]]))
Photon
energy
Water Bone Muscle
10 keV 0.91 3.5 0.93
100 keV 0.95 1.5 0.95
Diagnostic Radiology Part II : RadiationPhysics
32
-
8/9/2019 01 Radiation Units
33/41
Mean absorbed dose in a tissue or organ
• The mean absorbed dose in a tissue or
organ DT is the energy deposited in the
organ divided by the mass of that organ.
Diagnostic Radiology Part II : RadiationPhysics
33
-
8/9/2019 01 Radiation Units
34/41
• The equivalent dose H is the absorbed dosemultiplied by a dimensionless radiation weighting
factor, wR which expresses the biological
effectiveness of a given type of radiation
• To avoid confusion with the absorbed dose, the SI
unit of equivalent dose is called the Sievert (Sv). The
old unit was the “rem”
1 Sv = 100 rem
Equivalent dose: H
Diagnostic Radiology Part II : RadiationPhysics
34
-
8/9/2019 01 Radiation Units
35/41
Radiation weighting factor, wR
•
For most of the radiation used in medicine(X-rays, , e-) wR is = 1, so the absorbed
dose and the equivalent dose are
numerically equal
• The exceptions are:
– alpha particles (wR = 20)
– neutrons (wR = 5 - 20).
Diagnostic Radiology Part II : RadiationPhysics
35
-
8/9/2019 01 Radiation Units
36/41
Radiation weighting factor, wR
Diagnostic Radiology Part II : RadiationPhysics
36
-
8/9/2019 01 Radiation Units
37/41
Ti i h i f
-
8/9/2019 01 Radiation Units
38/41
Tissue weighting factors, wT
ORGAN /
TISSUE
WT ORGAN /
TISSUE
WT
Bone marrow 0.12 Lung 0.12
Bladder 0.05 Oesophagus 0.05
Bone surface 0.01 Skin 0.01
Breast 0.05 Stomach 0.12
Colon 0.12 Thyroid 0.05
Gonads 0.20 Remainder 0.05
Liver 0.05
Diagnostic Radiology Part II : RadiationPhysics
38
Eff i d E
-
8/9/2019 01 Radiation Units
39/41
Effective dose, E
E = T wT.HT
E : effective dose
wT : weighting factor for organ or tissue T
HT : equivalent dose in organ or tissue T
Diagnostic Radiology Part II : RadiationPhysics
39
-
8/9/2019 01 Radiation Units
40/41
Non-SI Units
Quantity Old Unit SI Unit Conversion
Activity curie (Ci) becquerel (Bq) 1 Ci=3.7 x 1010Bq
Absorbed
Dose rad gray (Gy) 1 rad = 0.01 Gy
Equivalent
Dose rem sievert (Sv) 1 rem = 0.01 Sv
-
8/9/2019 01 Radiation Units
41/41
41
In a mixed radiation environment, a person receives 20 mGy of
γ-ray dose and 2 mGy of slow neutron dose.
Calculate the total equivalent dose received by the person.