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Determining Dial Values

Because the dose calibrator is not inherently capable of detecting what radioisotope has been
inserted in the re-entrant detector, the user must select the isotope currently being measured.
Each radioisotope selection has a corresponding calibration value (also referred to as a dial
value for historical reasons). The dial value for a given isotope is a non-unit number that is a
means of expressing the detector’s sensitivity to the radiation produced by that isotope. Dial
values are defined relative to Co-60, an isotope selected for its high energy gamma radiation
and long-term stability. The detector response (R) for a given isotope is defined as the amount
of detector current produced by a given amount of isotope activity:

𝑅

!"#$#%&

!"#$

!"##$%&

!"#$#%&

!"#$%$#&

(1)

The dial value for Co-60 is defined as 5.0. The dial value for any other isotope is defined as 5.0
times the response of the detector to Co-60 divided by the response of the detector to the
isotope:

𝐷𝑖𝑎𝑙

𝑣𝑎𝑙𝑢𝑒

!"#$#%&

(2)

The response of the detector can be calculated as the sum of the detector sensitivity to photons
of energy e, times the intensity of the photon radiation of energy e from the radioisotope:

𝑅

!"#$#%&

(3)

Where the sensitivity of the detector to photons of energy e, in

𝑛𝐴

𝑚𝐶𝑖

is:

𝑆

!"#$%

!"##$%&

!"!#$

!"!#$%

(4)

A sensitivity curve can be made by plotting the sensitivity of a detector with respect to photon
energy. Given the photon sensitivity curve and the intensity of photon emission from
radioisotopes, which are tabulated in sources such as references 1 and 2, the detector response
and dial value can be calculated using equations (1), (2), and (3). The calculated dial values for
many common isotopes are listed in the appendix to this manual (see Appendix E: Calibration
Values).

The sensitivity curve of the Atomlab detector with respect to photon energy has been
determined by means of measuring NIST calibrated isotopes at a multitude of photon energies.
The isotopes: Ga-67, Tc-99m, In-111, I-125, I-131, and Tl-201 were provided from the NIST
standard reference materials program. In addition, NIST traceable sources of the isotopes F-18,
Co-57, Co-60, Ba-133, and Cs-137 were measured. All of the NIST SRM isotopes with the
exception of Xe-133 were contained liquid solutions of 5 ml volume in sealed borosilicate glass
ampoules with a wall thickness of 0.6mm. The Xe-133 samples were sealed, mixed with non-
radioactive xenon gas in borosilicate glass ampoules with a wall thickness of 1.3 mm. The F-18
source was measured in a 10 ml B-D plastic syringe. The Co-57, Co-60, Ba-133, and Cs-137
sources were contained within epoxy sealed type E-vials.

In addition to the isotopes measured, a Monte Carlo simulation of the Atomlab detector has
been created. The Monte Carlo simulation allows better determination of the shape of the
sensitivity curve in the regions between measured data points. The photon sensitivity curve
generated by the Monte Carlo simulation was found to be consistent with the sensitivity curve
generated by the isotope measurements. The Atomlab detector sensitivity curve is plotted
below with a logarithmic scale for photon energy: