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Specifications – SI Units

The International System (SI) units are a consistent set of units used in all branches of science. The International Commission on Radiation Units and Measurements (ICRU) has published its recommendations on the quantities and units to be used in the measurement of ionizing radiations and activity.

The quantity exposure rate is replaced by the quantity air kerma rate. The preferred SI units for air kerma rate are submultiples of Gray per second. A constant factor may be used to convert exposure rate to kerma rate. Quite simply, an exposure of 1 Roentgen per hour (1R/h becomes 2.425 microGray per second (µGy/s), or 8.73 milligray per hour (mGy/h).

Gamma radiation sources are specified in terms of exposure rate or air kerma rate, at a distance of 1 meter from the source. At this distance, the exposure rate strength of most sources will be in units of mR/h, and air kerma rate will be in units of µGy/h. A conversion factor of 8.73 should therefore be used to convert mR/h to µGy/h.

Radiation sources previously specified in terms of equivalent activity may be converted to the new quantity and units by first converting to exposure rate using the appropriate exposure rate constant.

Quick SI units reference guide for gamma radiation sources

 

Specification of output

  • New quantity: Air kerma rate
  • Old quantity: Exposure rate or equivalent activity exposure rate constant.
  • New unit: Gray (Gy) per second (s)
  • Old unit: Roentgen ® per hour (h)

Conversion of activity

  • Multiply value by 8.73 to convert from mR/hour to µGy/h

Specification of activity

  • Quantity: Activity (content)
  • New unit: Becqurel (Bq)
  • Old unit: Curie (Ci)

Activity Conversion

  • Multiply value by 37 to convert from kCi to TBq, Ci to GBq, or mCi to MBq
  • Multiply value by 27.03 to concert from MBq to µCi, GBq to mCi, or TBq to Ci

Radiation Protection

  • Quantity: Absorbed dose
  • New unit: Gray (Gy)
  • Old unit: rad
  • Quantity: Dose equivalent (biological dose)
  • New unit: Sievert (Sv)
  • Old unit: rem 

Conversion

  • Multiply value by 10 to convert from rad to mGy or rem to mSv
  • Multiply value by 100 to convert from Gy to rad or Sv to rem

 

Prefixes for units

Sub
Multiples
 
Mutiples
 
 
10-3
milli
m
103
kilo
k
10-6
micro
µ
106
mega
M
10-9
nano
n
109
giga
G
10-12
pico
p
1012
tera
T
10-15
femto
f
1015
peta
P
10-18
atto
a
1018
exa
E

 

Examples

Conversion of outputs for an equivalent activity of 1 Curie

Nuclide
Exposure rate at 1m*
Air kerma rate at 1m
Cs-137
0.33R/h
2.9mGy/h
Co-60
1.30R/h
11mGy/h

 *The value given is also the recommended exposure rate constant value in units of R m2 h-1 Ci -1
 

Examples of conversion for units of content activity

1Bq = 2.703 x 10-11 Ci
1nCi = 3.7 x 10Bq
1kBq = 2.703 x 10-8 Ci
1µCi = 3.7 x 104 Bq
1MBq = 2.703 x 10-5 Ci
1mCi = 3.7 x 107 Bq
1GBq = 2.703 x 10-2 Ci
1Ci = 3.7 x 1010 Bq
1TBq = 2.703 x 10Ci
1kCi = 3.7 x 1013 Bq

 

Daughter Nuclides

Some daughter nuclides may be in the equilibrium with the parent nuclide when source is supplied. In cases where this may occur, the transition probabilities for the daughter nuclides relate to disintegrations of each daughter.

Daughter nuclides with half-lives greater than the parent nuclide have not been listed since they would be present only in insignificant amounts.

Particular Energies

For ßemission, the end-point energy is quoted.

Transition Probabilities

These are expressed as percentages of the total number of nuclear transformations of the relevant nuclides. For electromagnetic transitions, the probability of photo emission has been listed.
 

Abbreviations

Half-lives
y – years
d – days
h – hours
min – minutes
s – seconds
ms – milliseconds
µs – microseconds
 
Type of Decay
e.c – electron capture
i.t. – isomeric transition
s.f. – spontaneous fission
 
Photons emitted
IC – indicates that photons of the stated energy are ~ 100% internally converted.
 
Calibration
For those sources that can be calibrated, certificates of measurement quote the results of air kerma at a specified date.
 
Calibration Uncertainty
The reported uncertainty is based on standard uncertainty multiplied by a coverage factor k=2, providing a level of confidence of approximately 95%.

Contact QSA for additional technical information on radiation units and measurements.