Re[2]: Calibration of a source

[TDC@ehssun.lbl.gov]

> --It would be very much cheaper to buy or borrow a small
>calibrated source and measure the Co-60 gamma ray peak with a NaI
>scintillation detector at various distances. This calibrates your
>detector, so it can then be used to measure your source. By varying the
>distances and counting times, you should be able to overcome the very
>large difference in source strength. A small calibrated source should not
>cost more than $100.

As practical as this may sound ...... A calibration such as this is only
valid if a Standard Instrument is used as the transfer standard - and an
NaI is NOT a Standard Instrument.  Even then - when you change the
distances you change the scatter component.  So - this is not a valid
transfer.

A Standard Instrument suitable for this purpose would be a condenser R
chamber.  There are others.  The is NOT to say that ANY ion chamber
instrument would qualify.  For instance an MDH does NOT qualify as a
standard instrument - although I HAVE seen companies mentioned here use one
for exactly this purpose.  Such is NOT a valid calibration as per ANSI
N323.

Actually - one could make an easier case of a GM instrument with a scaler
qualifying as a Standard Instrument - so long as you are dealing with a
same isotope, same geometry transfer.  However, there are those who would
debate this issue.  It WOULD meet the accuracy/precision criteria for a
Standard Instrument.