interaction of radiation with matter

 When X-rays and gamma-rays fall on an object, some of those photons interact with particles (electrons, protons) of matter, and their energy is either absorbed or scattered. This absorption or scattering of photon is called attenuation. The remaining photons go straight from the particle of any matter without interacting with it.

The number of photons that transmit from a material depends on the thickness, density, atomic number of the material and the energy of each photon.

There is a possibility of four types of reaction when there is an interaction with the matter of an ionizing radiation.

1. COHERENT SCATTERING -Radiography / Radiotherapy in ......... Read More
2. PHOTOELECTRIC EFFECT - It's an all or none ........ Read More
3. COMPTON EFFECT- Photon in this effect .......... Read More
4. PAIR PRODUCTION- It is in an energy mass ....... Read More

These four effects lead to the following conclusion -

When absorption is through the photoelectric effect, bone absorbs more energy than soft tissue (up to 6 times more).

Bone and soft tissue per gram absorb the same energy in the case of the Compton effect.

If absorption is only through Pair production, Bone absorbs twice as much energy than Soft Tissue.

Energy effect

UP to 50 keV Photo Electric Effect

60-90 KeV Photo Electric Effect + Compton Effect

200KeV-2 MeV Compton Effect

5 MeV-10 MeV Pair Production begins to important

50 MeV-100 MeV Pair Production most important

pair production

 Energy का mass में conversion ही एक pair production का उदाहरण हैं। 1.02 MeV इसके लिए threshold energy होती हैं। जब threshold energy वाला  photon nucleus के field के पास आता है तो यह गायब हो जाता हैं। जिसके कारण electron positron pair produce होता हैं। इसी प्रभाव को pair production कहते है।

 electron positron pair को उत्पन्न करने के लिए 1.02 MeV energy का उपयोग होता हैं तथा दोनों particles में शेष ऊर्जा समान रूप से स्थानांतरित हो जाती हैं। जिससे एक दूसरे  के opposite direction में दोनों ओर गति करते है।

 हालांकि 1.02 MeV threshold energy होती है pair production के लिए लेकिन दोनों particle को kinetic energy देने के लिए पर्याप्त energy न हो तो यह तब नही पाया जाता हैं। साधारणतः pair production 2 Mev से कम energy पर नही पाया जाता हैं। atomic number (z) पर pair production निर्भर करता है।

Pair Production ∝ Z2

equivalent dose in radiology

However , the absorbed dose in a medium for all ionizing radiation is measured in energy joules per kilogram ( J / kg ) absorbed by per unit mass . But the biologiacl effect produced by the same or equivalent dose of different types of radiation is  not the same. 

kinetic energy release in medium ( KERMA )

Initial interaction of an X-Ray photn in a medium is  called KERMA. When X and 4-rays pass through a medium, they transfer their kinetic energy to the charged particle electrons and protons. KERMA (K)  refers to the energy transferred to the charged particle. This charge trajectory forward into the particle medium

radiation exposure

Exposure is the amount of ionizing photon present in a radiation beam. The amount of ionizing photon in a medium is determined by the number of ions generated by them. Air is an universal medium for measuring exposure.

A Roentgen X or Gamma is the amount of radiation through which 0.001293 grams of air-entrapped corpuscular emission can flow 1esu of any sign.

unit of radioactivity

The unstable nucleus number present in a radionuclide is called activity. This nucleus radio-disintegraion

Achieve stability through Radiation emanating from a radioactive material is directly related to its activity

Is proportional.

Therefore, the activity shows the amount of radiation emitted in a unit time. Its unit curie (Ci)

 It happens.

A curie denotes the disintegration per second or dissolution per second that occurs in 1 gram of radium 226.

1Ci = 3.7 X 10 ¹⁰ dps

Its small units, milli curie and microcurry are also used.

1mCi = 3.7 X 10 ⁷ dps

 1μCi = 3.7 X 10 ⁴ dps

principle of radiation measurement

ionization

Radiation creates positive and negative ions in a gas or matter, this process is called Ionization. This process is caused by the release of electrons from ions or molecules. These ions which are two in number are called Ion Pair, and can be collected by applying an electric field, which is found in the form of curent or puslses. The total charge collected is proportional to radiation intensity. Generally gaseous or solid media is used for Ionization. Ionization can be used in Bq measurement and dose measurement. Ionization Chamber, Proportional Counter, Geiger-Muller (GM) Counter and Semiconductor Detector etc. fall under this category.

farmer chamber

Compton effect

In the Compton effect , the photon interacts with the free electron of the atom and gives it only some of its energy as orbit (E _b ) and Kinetic Energy (E _K) . Incidnet photon with the remaining {Energy H𝒗 - (E _B + E _K )} goes in a different direction and the other atom interaction is.

Compton effect

 photo electric effect

It is an all or none phenomenon  . In this interaction , the incident photon orbit interacts with the bound electron. At this time a portion of the photon's energy is used to drive the electron (BE = E _B ) out of the orbit, and the remaining energy (h𝒗 - E _B ) is given to the electron as kinetic energy . 

photo electric effect

Note  

Bound electron

When the energy of the photon is equal to or slightly higher than that of the interating electron, it is called bound or bound electron. 

Free electron

When the energy of photon is much higher than the interacting electron, it is called free electron.

coherent scattering 

This effect is not very important in Radiotheray / Radiology . When the interacting photon passes through the orbital electron of the target atom, it starts occluding. This results in the emission of a recurring energy similar to the incident photon. 

coherent scattering

Note  

Bound electron

Free electron

When Radiation is applied from outside the body in the form of a beam, this situation is called External Beam Radiotherapy . Apart from this, Radiation can also be given by placing Rdioactive source inside the body,   says Intracavitory / Intraluminal Radiotherapy .