9. Cobalt Leakage Limitss

Leakage limits Recap for 60Co & Linacs

Cobalt leakage limits with beam ON

0.1% of the primary beam at 1 meter distance

Linac leakage limits with beam ON

0.1% of the primary beam at 1 meter distance (same as cobalt)

What would be the maximum allowable leakage from a Cobalt source that emitted 193.3cGy/min?

0.1933cGy/ min (move decimal 3 places to the left)

A beam stopper is not to exceed __% of useful beam

0.1%

Transmission with beam OFF.

For linacs

zero

Transmission with beam OFF: for cobalt

We take readings at 14 different locations all 1 meter from the source

• Dose shall not exceed:

  • 2mR/ hr average

  • 10 mR/ hr in any one direction

Cobalt and Cesium sources for teletherapy must be put on what to prevent leakage? What do they leak?

double encapsulated in stainless steel

beta particles

T/F: Since Cobalt and Cesium sources for teletherapy are double encapsulated they do not need to be tested for leakage

False, they do

When do cobalt and cesium sources used for teletherapy tested for leakage

1. by manufacturer before delivery 

2. by hospital upon delivery 

3. then every 6 months

What type of test is done to check for leakage of cobalt sources for teletherapy?

leak test aka wipe test

Steps for a Leak Test/ Wipe Test

1. Wipe collimator leaves with a long-handle cotton applicator moistened in a detergent solution

2. Measure the cotton applicator with a GM counter

FYI: the leakage, beta particles, will be transferred to the cotton applicator so you test that

Who licenses Co60 sources?

NRC

Calibration for Co60:

1. Spot checks

2. Full calibration

monthly spot checks

yearly calibration