[ RadSafe ] Mutual Effects of U and T factors- Linac Room

kali1 at zahav.net.il kali1 at zahav.net.il
Sun Feb 3 12:38:07 CST 2008

If the calculated dose rate is less than 2 mrem in any one hour and the 
annual dose calculated is lower than 100 mrem, you should not be worried 
about IDR. You should also remember that the recommendations of NCRP 151 are 
quite conservative and when you calculate 100 mrem per year, the real dose 
is probably more like 10-15 mrem. All these dose values are quite 

Yehoshua Kalish,
Hod-Hasharon, Israel

----- Original Message ----- 
From: "Moshe Levita" <mlevita at tasmc.health.gov.il>
To: <radsafe at radlab.nl>
Sent: Sunday, February 03, 2008 6:45 PM
Subject: [ RadSafe ] Mutual Effects of U and T factors- Linac Room

Dear Colleagues,

In designing Linear Accelerator rooms one is considering the Use factor U
and Occupancy factor T .

Under certain conditions the multiplication of these two factors can be
very small.

Imagine 2 energies machine and very high leakage when the machine is 
operating in its
high energy mode AND maximal field size AND pointing at specific direction.


The high energy fraction is         :   40  %  of total treatments

Direction Factor is                       :   10  %  of all directions

Maximal field size fraction is        :   20  %  of all treatments

Then, Total (Use Factor) = 1/125

If the relevant Occupancy Factor is  : 1/16   at uncontrolled areas - 
partial time

Then Use x Occ. Factors = 0.0005

These small factors might allow very high I.D.R (Instantaneous Dose Rate).

Though many experts doesn't care about limiting the I.D.R,  I would like to

ask your opinion about any known limitation on the Use x Occ. Factors ?

e.g why not limiting the  Use x Occ. Factors to be > 1/64 or other value ?

Thank you,

Moshe Levita
Chief Physicist
Tel Aviv Medical Center

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