Artificial Radioactivity And Qvalue Consider the nuclear reaction 11 Na 23 α 13 Al 27 12 Mg 26 1 H 1 Z X A α Z2 C n ID: 464900
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Slide1
Artificial RadioactivitySlide2
Artificial Radioactivity And Q-value
Consider the nuclear reaction:
11
Na
23
+ α → [
13
Al
27
] →
12
Mg
26
+
1
H
1
Z
X
A
+ α → [
Z+2
C
n
A+4
] →
Z+1
Y
A+3
+
1
H
1
Target Compound Product
nuclei
The above reaction is an example for (α, P) reactions.Slide3
Consider a nuclear reaction represented by the equation:
x + X → Y + y
Bombarding
Target Product
Product
particle nucleus nucleus particleAssume that the target nucleus is at rest and has no K.E.The total energy of a particle atom = R.E + K.E. Rest energySlide4
(E
x
+ m
x
C
2) + MXC2 = (EY + MYC2) + (E
y + myC2) (1)
K.E R.E R.E K.E R.E K.E R.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 Q-value.
Q = EY + Ey – Ex
In terms of the masses: EY +
Ey – Ex = (MX +
m
x
– MY – my) C2 From equation (1): Q = EY + Ey – Ex = (MX + mx – MY – my) C2Slide5
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 N
14, the mass of the neutron can be obtained (Chadwick).Slide6
The transuranium elements
Artificial radioactivity can be achieved not only with isotopes of elements but also with
transuranium
elements.
These are elements of atomic no. greater than 92.These elements have been generated by bombarding U (238) with neutrons or α-particles.
EX: 92U238 +
0n1 → (92U
239)* + γ (
92U239)* →
93Np239 + -1β0
; Np: Neobium.
(
93Np239)* → 94Pu239 + -1β0 ; Pu: Plutonium.Slide7
All of the
transuranium
elements are radioactive, all are α-emitters and some both α-emitters 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. Slide8
Alpha decaySlide9
1- The velocity and energy of alpha particles:
The method that gives the velocity and energy of α-particles 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 / MSlide10
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 Mv
2
Slide11
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 = MrV
r Mass of Its Mass of Product
α-particle velocity product velocitySlide12
→
V
r
= (M /
Mr) V → (1) The α-disintegration energy is: Eα = 1/2 MV2 + 1/2 Mr
Vr2
Substitute from equation (1): Eα
= 1/2 MV2 + 1/2 Mr [(M2
/Mr2) V2]
Eα = 1/2 MV2
[1 + (M/Mr)] Slide13
Interaction of Radiation with MatterSlide14
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.Slide15
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.Slide16Slide17Slide18
Mechanisms of Charged-Particle Energy Loss
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.Slide19
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).
Slide20
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 X-radiation 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. Slide21
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 radiationSlide22
In other words:
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.