Brain Teaser #15

A) Question:

What is the half-life of cobalt-60?

A) Answer:

The half life of cobalt-60 is 5.27 years.

B) Question:

Does cobalt-60 give off photons? (yes or no)

B) Answer:

Yes, cobalt-60 gives off photons.

C) Question:

How many photon energies are predominantly emitted?

C) Answer:

2 photon energies are predominantly given off.

D) Question:

What is the average energy given off by cobalt-60 from the photons?

D) Answer:

1.25 MeV is the average energy given off by cobalt-60.

Follow Up:

Since cobalt-60 gives off two photon energies, one photon has an energy of 1.173 MeV and the other has an energy of 1.332 MeV. If you add these two energies together you get 2.505 MeV. Take this number and divide by 2. This will give you the average photon energy of 1.25 MeV

1.173 + 1.332 = 2.505

------------------------------------------------------ = 1.25

E) Question:

What is the most common SSD used for a cobalt-60 unit?

E) Answer:

The most common SSD for a cobalt-60 unit is 80 cm.

F) Question:

Why would it be advantageous to use an SSD of 80 cm instead of 100 cm for a cobalt-60 unit?

F) Answer:

80 cm is usually advantageous over 100 cm for a cobalt-60 unit because 80 cm will decrease the time that the unit is in the on position when compared to 100 cm.

G) Question:

If you did a calculation for a cobalt-60 unit and you figured out that the machine had to be on for 60 seconds (for an 80 cm SSD), if the SSD was changed to 100 cm, please give an approximate new time if all other variables remained the same.

G) Answer:

The approximate time would be 94 seconds if you changed the calculation for 100 cm SSD.

Follow Up:

In the real world their may be other variables that would change if you increase your SSD, the main factor to worry about is the distance factor (which is governed by the Inverse Square Law). This is simply the new SSD squared divided by the old SSD squared times 60 seconds.

(new SSD)2
----------------------------------------------------------------------------------------------------- x old time = new time
(old SSD)2


(100 cm)2
---------------------------------------------------------------------------------------------------- x 60 seconds = 94 seconds
(80 cm)2

H) Question:

If the cobalt-60 source increases in size, will your penumbra increase, decrease, or remain the same?

H) Answer:

If the cobalt-60 source increases in size, then the penumbra will also increase.

I) Question:

If 300 cGy is given to the 60% line for a single direct field what would the d-max dose be?

I) Answer:

The d-max dose would be 500 cGy if 300 cGy was given to the 60% line.

Follow Up:

D-max dose is the maximum dose (or 100% of the dose) so, if you give dose to the 60% line then the 100% line or d-max will get that much more dose. To figure this out mathematically, simply take 300 and dived it by .6 (you can also set up a proportion. I will show both ways):

300cGy --------------------------------------------------- = 500 cGy

  300 cGy                   X
      ---------       =       ---------
     60%                   100%

x = 500 cGy

Mathematically, these equations are the same thing. The proportion is easier to see. When in doubt, try to set up a proportion to solve something.


Author: Adam Buell
Co-Author: Deb Zehel
Editors: Mary Hare
Date Created: Febuary 2006