When it comes to photons radiation protection, we should not only consider the originally incident photos but also should take care of the scattered photons and characteristic x-rays that follow because these following photons contribute to the dose as well. A build up factor is called for counting this fact.
Build up factor depends on (1)source (photons) energy (2)the distance traveled and (3)the nature of the medium. With build up factor, we could calculate the dose more precisely.
In order to satisfy the Requirements for structural shielding for X-ray facilities, we consider the following factors. (1)the maximum voltage that X-ray tube is used. This is related to the incident photos energy. (2)maximum current(mA) of beam current, which relate to flux of radiation. (3)work load of the X-ray tube. (4)the use factor. The fraction of workload that the beam is pointed directly to. (5)the occupancy factor.
By operating integration of energy over E_g to E_g-1 on energy dependent neutron diffusion equation, we can get definitions of some group constant like group absorption cross section, group scattering cross section weighed by flux.
For diffusion coefficient, it's weighed by gradient of flux, not flux itself. It is reasonable because the diffusion depends on gradient of flux, not flux.
It's interesting that we have flux dependence( or gradient of flux) , which we have not known yet, in the definition of group constants. This is called a recursive problem. To tackle this, we assume the spatial part and energy part of flux is separable. And then, we further assume that we know the energy dependence. For example, we assume a Maxwellian-Boltzmann distribution for thermal neutrons.
Since the difficulty of group parameter constants are already solved, we can easily write down the two group diffusion equation. What following is similar in one group. We will have two kind of problems, source problems and criticality problems.
Starting from expanding flux dependence to even include direction and energy. ø( position, energy, direction, time), we write down the neutron balance equation. And then, omit direction dependence first and focus on energy dependence from now on.
ø( vector r, E, t)
Notice that not only flux has a energy dependence, some parameters like cross section, velocity or diffusion coefficients are also energy dependent now. Some changes in neutron balance equation: 1. Fission neutrons come from fission reactions of neutrons of all energy. 2. Losing neutrons by scattering from objective energy to any other energy. 3. Gaining neutrons by scattering from any other energy to objective energy.
Then we define g group of flux ø_g by integrate the flux from E_g to E_g-1. By this, we have the neutron balance equation of group g.
For example, we had taken a look of two group, G=2. Some simplifications are used like up-scattering, fission neutrons being all fast
These weeks, I found I took the notes too slowly to have them done in time in class.
I wondered if it's just a coincidence but I happened to sit on the back seats when it happened.
Why the difference?
The only explanation coming across my mind is the TRAVELING of LIGHT!
Since the front seats are much closer to the blackboard, the time the light needed to travel from blackboard to my eyes, positioned in the front seats, is apparently less then it is for the back seats. I need more time to gather information on blackboard if I choose the back seats and that's why I took notes more slowly.
Time flies. scaring..... About one month later, I will jet back to Taiwan. That also means that it's almost a year for my studying abroad. Sigh.......... I would only say it's the busiest year of my life. However, there is not too much exciting new things knocking my brain. Not much research, not much inspiring ideas.... Most courses I took this year were already taken in Taiwan before. Just a improved skill and a better feeling of the subject. I hope I could take more challenging courses next semester. On the other hand, I found that I studied faster now, but think things less. Do keep in mind that thinking and curiosity is the most important.