Transcript Artificial Radioactivity

ARTIFICIAL RADIOACTIVITY
ARTIFICIAL RADIOACTIVITY AND Q-VALUE
Consider the nuclear reaction:
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23
11Na
+ α → [13Al27] → 12Mg26 + 1H1
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A +α→[
A+4] →
A+3 + H1
X
C
Y
Z
Z+2 n
Z+1
1
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Target
Compound
nuclei 
Product
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The above reaction is an example for (α, P) reactions.
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Consider a nuclear reaction represented by the 
equation:
x+X→Y+y
Bombarding
Target
Product
Product 
particle
nucleus
nucleus
particle 
Assume that the target nucleus is at rest and has no K.E.
The total energy of a particle atom = R.E + K.E. 
Rest energy
(Ex + mxC2) + MXC2 = (EY + MYC2) + (Ey + myC2)
R.E
K.E
R.E
R.E
K.E
R.E
(1)
K.E
The difference between K.E of the products of the reaction and that of
the incident particle is called the energy balance of the reaction or Qvalue.
Q = EY + Ey – Ex
In terms of the masses:
EY + Ey – Ex = (MX +mx – MY – my) C2
From equation (1):
Q = EY + Ey – Ex = (MX + mx – MY – my) C2
If the value of Q is +ve, the K.E of the products is greater
than that of the reactant, and the reaction is said to be
exothermic or exogeric.
If Q is -ve, the reaction is endothermic or endoergic.
The Q-value is one of the main sources of information
about nuclear properties.
From the Q-value and the known atomic mass of B11, He4,
and N14, the mass of the neutron can be obtained
(Chadwick).
THE TRANSURANIUM ELEMENTS
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Artificial radioactivity can be achieved not only with isotopes of elements but
also with transuranium elements.
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These are elements of atomic no. greater than 92.
These elements have been generated by bombarding U (238) with neutrons
or α-particles.
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EX:
92U
238
+ 0n1 → (92U239)* + γ
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(92U239)* → 93Np239 + -1β0
; Np: Neobium.
(93Np239)* → 94Pu239 + -1β0
; Pu: Plutonium.
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All of the transuranium elements are
radioactive, all are α-emitters and some both αemitters and β-emitters.
The most important of these elements from the
point of view of nuclear engineering is
plutonium (239) because of its usefulness as a
nuclear fuel.
ALPHA DECAY
1- THE VELOCITY AND ENERGY OF ALPHA
PARTICLES:
The method that gives the velocity and energy of αparticles depends on the measurement of the defection
of the paths of the particles in a magnetic field.
When a charged particle moves in a magnetic field, its
orbit is a circle whose radius is determined from the
relation:
qBv = Mv2 / r
Also,
v = qBr / M
The velocity can be determined if the strength
of the magnetic field is known and if the radius
of the orbit is measured.
 Also,
the k.e = 1/2 Mv2
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2- NUCLEAR ENERGY LEVELS (DISINTEGRATION
ENERGY):
The total energy change in an α-decay process is
called α-disintegration energy.
When an α-particle is emitted, the product, or
residual nucleus, carrying with it a certain amount
of energy.
The α-disintegration energy is the sum of kinetic
energies of the α-particle and the product nucleus,
and is found as follows:
From the low of conservation of momentum:
MV = MrVr
Mass of
Its
α-particle velocity
Mass of
product
Product
velocity
→ Vr = (M / Mr) V → (1)
The α-disintegration energy is:
Eα = 1/2 MV2 + 1/2 MrVr2
Substitute from equation (1):
Eα = 1/2 MV2 + 1/2 Mr [(M2/Mr2) V2]
Eα = 1/2 MV2 [1 + (M/Mr)]
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INTERACTION OF RADIATION WITH MATTER
The interaction of radiation with matter depends on:
i) The type and energy of the incident radiation.
ii) The chemical and physical properties of the target material.
iii) The manner in which the incident radiation interacts with
the material.
This section contains the mechanisms by which ionizing
radiation interacts and loses energy as it moves through
matter. This subject is extremely important for radiation
measurements because the detection of radiation is based
on its interactions and the energy deposited in the material
of which the detector is made. Therefore, to be able to build
detectors and interpret the results of the measurement, we
need to know how radiation interacts and what the
consequences are of the various interactions.
For the discussion that follows, ionizing radiation is
divided into three groups:
 Charged particles: electrons (e-), positrons (e+),
protons (p), deutero- ns (d) , alphas (α), heavy ions
(A > 4).
 Photons: gammas (γ) or X-rays.
 Non-charged particles: Neutrons (n).
 This classification is convenient because each
group has its own characteristic properties and
can be studied separately.
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Ionizing
Radiation
Charged
Particles
Photons (X-ray,
γ-ray)
Heavy Particles
Light Particles
P, α, heavy ions
e- , e +
Neutrons
MECHANISMS OF CHARGED-PARTICLE ENERGY
LOSS
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Heavy charged particles, such as alpha particle,
interact with matter primarily through coulomb
forces between their positive charge and the
negative charge of the orbital electrons of the
absorbed atoms. Although interactions of the
particle with nuclei as in Rutherford scattering or
alpha particle induced reactions are also possible,
such encounters occur only rarely and they are not
normally significant in the response of radiation
detectors. Instead, charged particle detectors
must rely on the results of interactions with
electrons for their response.
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Upon entering any absorbing medium, the charged
particle immediately interacts simultaneously with
many electrons. The electron feels an impulse
from the attractive coulomb force as the particle
passes its vicinity. This impulse may be sufficient
either to raise the electron to a higher-lying shell
within the absorber atom (excitation) or to remove
the electron from the atom (ionization).
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On the other hand, when energetic electrons penetrate
materials, they lose energy by two mechanisms:
Collision loss, where energy is given to electrons in the
atoms of the material, and radiation loss involving the
conversion of electron kinetic energy to photons of Xradiation in the field of an atomic nucleus. As the
incident electron traveling through the material, it might
pass by a particular atom in about 10-18 s so it is able to
exert large coulomb forces on the atomic electrons and
impart energy to them. The energy transfer may be
sufficient to allow the electron to leave the parent atom,
and so cause ionization, which is completed within about
10-15 s. Alternatively, the atomic electron may be excited
to a higher state. These inelastic collisions are the most
important mechanisms of energy loss.
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Emission of radiation is a mechanism for electron
energy loss at high kinetic energies where the
electron behavior is relativistic. An electron in the
electrostatic field of an atomic nucleus can
experience a large acceleration. The rate of energy
loss due to radiation increases with the atomic
number of the absorbing material and the kinetic
energy of the electron. This energy is lost in the
form of a photon of X-radiation, often referred to
as bremsstrahlung, or braking radiation
IN OTHER WORDS:
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Charged particles traveling through matter lose energy
in the following ways:
In Coulomb interactions with electrons and nuclei.
By
emission
of
electromagnetic
radiation
(bremsstrahlung).
By emission of Cerenkov radiation. Cerenkov radiation is
visible electromagnetic radiation emitted by particles
traveling in a medium, with speed greater than the
speed of light in that medium. It constitutes a very small
fraction of energy loss.