Category: Radiation protection

Transport Index ☢️🚚

ThirumuruganLeave a comment

Transport index is the maximum radiation level in mSv/hr, at a distance of 1 meter from the external surface of the package, multiplied by 100

TI = Radiation level at 1m (mSv/hr) x 100

TI \displaystyle \large \leq 0.05 can be considered as 0

The packages can be categorized based on the Transport Index

What is package?
The packaging with its radioactive content as presented for transport

what is packaging?
The assembly of components necessary to enclose the radioactive contents completely

Packaging + radioactive content = package

Packages are categorized based on TI into three categories
I – white
II – yellow
III – yellow
III – yellow (exclusive use)

Transport IndexMaximum dose rate on the surfacecategory
0upto 5 µSv/hrI – white
> 0 to 1> 5 µSv/hr to 0.5 mSv/hrII – yellow
> 1 to 10> 0.5 mSv/hr to 2 mSv/hrIII – yellow
> 10> 2 to mSv/hr to 10 mSv/hrIII -yellow
(exclusive use)
Categories of packages

UN classification number: class 7 for radioactive materials

Reference

Rajan, KN Govinda. Radiation Safety in Radiation Oncology. CRC Press, 2017.

why do we use materials with low atomic number for shielding against neutrons 🤔

ThirumuruganLeave a comment

Types of Neutron Interaction

The neutron interaction is one among the major types Scattering and absorption.
In case of scattering, when the neutron interact with the nucleus the speed and direction of the neutron changes but the nucleus will be left with same number of neutron and proton as before. The energy imparted to the nucleus depends upon the angle of scattered neutron and mass of the nucleus. Eventually the nucleus will have some recoil velocity and will be in excited state that will result in the release of radiation.

The Scattering is subdivided into two types

  1. Elastic scattering
  2. Inelastic Scattering

Elastic Scattering

Elastic scattering is a dominant mechanism of energy deposition in case of High energy neutrons. During the interaction, as discussed earlier the fraction of neutron kinetic energy in transferred to the nucleus. The average energy loss for a neutron interacting with the nucleus of atomic weight A is \displaystyle \large \bf{\frac {2EA}{(A+1)^2}} from this expression its clearly understood that we need to use a material with low atomic number.

If we use hydrogen whose atomic weight A = 1 , the average energy lost will have highest value of E/2. i.e. if the neutron is 2 MeV , because of one single elastic collision it loses energy in average about 1 MeV. and in next collision in average it loses 0.5 MeV. It almost take 27 collision in this example before in becomes thermal neutrons i.e. 0.025 eV. As the atomic number is low as well as the mass of hydrogen is comparable to neutrons. The number of interactions to reduce the energy of neutron to desired level is less

Equation to find number of collision(n) in average required to reduce initial energy of neutron Eo to the desired energy En is

\displaystyle \huge \bf{\frac{log(E_n / E_0)}{log[(A^2 +1)/(A+1)^2]}}

Inelastic Scattering

Inelastic scattering is same as that of elastic scattering expect here the nucleus undergoes internal rearrangement to go to excited state which eventually results in emission of radiation. The kinetic energy of the outgoing nucleus + neutron is less than that of incoming neutrons K.E. because some energy is lost in making the internal rearrangement of nucleus.

If the energy to make the nucleus go to excited state is very high it is highly unlikely that the inelastic collision takes place. It is the same case with hydrogen nucleus, they do not have excited states. so inelastic collisions are not possible. In general the scattering reduces(moderates) the energy of neutrons

Absorption

In case of low energy neutrons, the elastic scattering is not possible instead absorption or capture of neutrons takes place(, where the nucleus may rearrange its internal and emit one or more gamma rays or other charged particles such as protons, deuterons and alpha particles. the nucleus may also releases excess neutrons to become stable. The absorption of energy is greater than the energy of the emitted charged particles and neutrons.

Conclusion

These are the reasons why materials with low atomic numbers are used for neutron shielding which is contradictory to what we do for high energy photons, where we use high atomic number materials.

This is the reason why we use concrete barriers with combination of hydrogenous materials for construction of linac bunkers as they can shield photons, photo neutrons, and other charged particles and gamma rays that are produced while moderating neutrons. e.g. Neutron capture in hydrogen releases 2.224 MeV gamma rays.

We will look at the shielding calculations in future posts

If you want to ask any doubt or if you find any corrections to be made in this post please leave a comment below. thanks 🙂

Reference

Rinard, P. “Neutron interactions with matter.” Passive nondestructive assay of nuclear materials 375-377 (1991).