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Physical of Nuclear Medicine
The radioactive element :It is unstable nuclei that disintegrate to emit various rays
and particles, such as :1- The Alph (α) particles :
Are positively charged, helium nucleus, 24 He have
range of energy stop in a few cm of air.
2- Beta particles (β) :
Are two kind :
a- β- are negatively charged are called negatron,
are more penetrating but can be stooped in a
few meters of air or few millimeters of tissue,
they are high – speed electrons.
b- β+ positive beta or positron are produced by
cyclotrons. β+ is physically identical to an
electron except that it has a positive charge.
3- γ – rays Electromagnetic radiation, identical to χ –
ray but γ – rays have much higher energies and they
are very penetrating than α and beta, α and γ rays
from a given source have fixed energies, but β have
a continuous of energies up to a maximum
characteristic of the source (fixed energies).
Isotopes:
Nuclei of a given element with different numbers of
neutron :
Stable isotopes
Isotopes
Unstable isotopes
- Stable isotopes :
They are not radioactive like 12C , 13C )
- Unstable isotopes :
They are radio active nuclei like [11C , 14C and 13C)
most elements do not have naturally occurring
radioisotopes but all produced artificially.
Nuclear reactor and nuclear medicine :
- The production of many artificial radio nuclides.
- In medicine used for:
Research , diagnostic and therapy from the cancer ,
tumor , …………
- γ – ray in medicine :
γ – rays are very penetrating, γ emitting radioactive
element inside the body can be detected outside the
body.
- Use less 1g of radio nuclides element in medicine
purpose.
The Radioactive decay and units of radioactivity :
A
The symbol of radioactive element is Z X where the :
X
is the element
A
atomic mass [total number of nucleons)
A=p+n
p = proton
n = neutron
Z
atomic number
Z=e=p
n=A–Z
number of neutron
If n < normal
unstable
radioactive
nucleic
γ , α , β rays
stable.
Radioactive decay :
Decay the nuclei of unstable isotopes to its daughter,
which is also radioactive, is formed each daughter decays
until the final daughter (stable isotope) after emitting the
radioactive rays (α , β and γ)
A
Z
‫ــ‬
decay
X 24He  ZA‫ ــ‬y
A‫ = ــ‬A – 4
Z‫ = ــ‬Z – 2
Ex :
27
When 13 Al is bombarded by deuterons, α – particles are
emitted. How many protons and neutrons does its
nucleus contain.
Solution :
A deuteron is the nucleus of the isotopes of H2of mass 2,
the equation of the reaction is :
27
2
4
A
Al

H

He

13
1
2
ZX
A + 4 = 29
Z + 2 = 14
A = 25 , Z = 12
25
the atom of 12 X is 1225 Mg
Ex :
α
γ ,β
β,γ
α
234
234
U  234
90Th 91 Pa 90 U  Th
234
92
206Pb( stabe)
Each radio nuclide decays at a fixed rate indicate by the
half life T1/2.
T1/2 : the time needed for half of the radioactive nuclei to
decay.
The basic equation describing radioactive decay is:
A = A0 e-λt
A = the activity in disintegration per second
A0 = the initial activity
λ = decay constant
t = is the time of decay
A=λN
N = the number of radioactive atoms N = N0 e-λt
T1/2 = 0.693 / λ
λ = 0.693 / T1/2
(a) activity %
A0
A0/2
Time (hr)
0
T1/2
The unit of radioactivity :
The Curie (ci) = 3.7 × 1010 disintegration / s
mci = 10-3 ci , ci = 10-6 ci
, nci = 10-9 ci
1CRU = international SI unit is Becquerel (Bq)
Bq = disintegration / sec
1 ci = 3.7 × 1010 Bq
KBq = 103 disintegration / sec MBq = 106 disi / sec
Ex :
1g of K40 (40K) to emit 105 B/sec what is  and T1/2 of
40
K.
Solution :
40g and 40K contains 6.02 × 1023 Avogardo's number
Potassium atoms.
1g contains 6.02 10 atom
23
40

A
10 3

 6.7  10 18 Sec
N
1.5  10 23
T1 / 2 
0.693
 1017 Sec 1
18
6.7 10
Year = 3.15 × 107 Sec
Ex : What is the decay constant of 131I in the Thyroid if
T1/2 = 1s days.
Ex : If a radio nuclide has a decay constant  of 0.001
days-1
a- T1/2
b- How much do have left after 24hr if you have
10MBq at t = 0