Chapitre-1 Sources de Rayonnements-En.pptx

Radiations sources

1. Radiation sources All kinds of radiation emitted by an atomic process or nuclear process cannot be outside of four types of radiation which are classified as follows: Charged particle radiations that encompass two types of radiation: Fast electrons; example: b + and b - radiation Heavy charged particles; example: radioactivity a (He ++ ) and fission fragments, Uncharged radiations which encompass two types of radiation: Electromagnetic radiation; example: UV, X-ray and gamma radiation. Neutrons; example: thermal, epithermal and fast neutrons.

2. Units of measurement Unit of activity from a radioactive source A= l N where N is the population of a type of radioactive nuclei l : radioactive decay constant l= l n 2/ T with T: half-life period A is expressed in disintegration /s= Bq 1Ci=3.7x10 10 Bq

3. Energy of radiation emitted by a source The energy of radiation in the form of particles represents its energy kinetics which E c =1/2mV 2 ( case classic E c <<mc 2 ) case : The energy of a radiation that has no mass represents its energy total E= h n = hc /l The unit used in radiation East the electronvolt , 1eV=1.6x10 -19 joules.  

4. Fast electron sources b decay : a radioactive nucleus (natural or artificial) can emit a positron if it has an excess of protons, we therefore speak of b + radioactivity and if it has an excess of neutrons it disintegrates by b - emission . Example of b - pure sources:

5. b- and b + radiations are characterized by a broad spectrum showing an energy distribution ranging from 0 to the maximum energy ( Endpoint Energy ) equal to the balance Q b .

6. Conversion electrons: An excited nucleus, by de-exciting itself, can transfer its energy of excitation E ex to an electron from an atomic orbital and thus eject it with a kinetic energy: E e = E ex -E b where E b is the binding energy of the electron in its atomic orbit.

7. Sources of alpha particles ( a) : Natural or artificial radioactive nuclei can decay by the emission of a helium nucleus or alpha particle of a certain energy which depends on the initial and final state of the disintegration reaction, namely: With  

8. Examples of alpha sources:

9. Sources of electromagnetic radiation: Ionizing electromagnetic radiation is: UV (10eV to 100eV), X-rays (1keV to 100keV) and gamma rays (>100 keV ). All 3 types of radiation can be produced by the braking of charged particles in matter or because of their curvilinear trajectories; 1st example : X-ray machines used in radiography in hospitals or in baggage control. 2nd example: linear electron accelerator with an energy of several MeV which strike a metal target thus producing a continuous spectrum of gamma rays of several MeV (called Bremsstrahlüng ) ; these gamma rays can be used in several fields such as radiotherapy ( LINAC accelerator at Blida hospital). X-rays and UV rays can also be produced by atomic excitation followed by emission of radiation characteristic of the emitting elements ( anti-cathode ).

10. Gamma rays are also emitted by excitation and de-excitation of the nucleus. Example: radioactive sources which disintegrate by beta or alpha emission leaving the daughter nucleus in an excited state which de-excites by emitting a gamma ray.

11. Neutron sources: Neutrons can only be emitted by a nuclear reaction caused by the interaction between 2 nuclei or a gamma ray and a nucleus or by nuclear fission. Examples of Neutron Sources: Sources of nuclear fission caused by a neutron ( Nuclear reactor ). Spontaneous fission sources (the Californium 252 Cf source). Neutron sources due to alpha radioactivity (isotopic sources): In fact, we mix an alpha-emitting radioactive substance with beryllium 9 Be and thus obtain a source of neutrons which are produced by the nuclear reaction:  

12. Sources of neutrons produced by a nuclear reaction induced by a gamma ray: in fact, a beryllium 9 Be or deuterium 2 H (D) target is exposed to gamma radiation which causes a neutron to be torn off and we thus obtain a source of neutrons which are produced by the following nuclear reaction: