A1 and A2 Values for Radionuclides

I. Single Radionuclides

  • 1. For a single radionuclide of known identity, the values of A1 and A2 are taken from Table 1 if listed there. The values A1 and A2 in Table 1 are also applicable for the radionuclide contained in (α,n) or (γ,n) neutron sources.
  • 2. For any single radionuclide whose identity is known but which is not listed in Table 1, the value of A1 and A2 are determined according to the following procedure.
    • (a) If the radionuclide emits only one type of radiation, A1 is determined according to the following method. For radionuclides emitting different kinds of radiation, A1 is the most restrictive value of those determined for each kind of radiation. However, in either case, A1 is restricted to a maximum of 1000 curies (37 TBq). If a parent nuclide decays into a shorter lived daughter with a half-life not greater than 10 days, A1 is calculated for both the parent and the daughter, and the more limiting of the two values is assigned to the parent nuclide.
      • (1) For gamma emitters, A1 is determined by the expression:
        A1 expression for gamma emitters

        Where Γ is the gamma-ray constant, corresponding to the dose in roentgens per curie-hour at 1 meter, and the number 9 results from the choice of 1 rem per hour at a distance of 3 meters as the reference dose-equivalent rate.

      • (2) For x-ray emitters, A1 is determined by the atomic number of the nuclide:
        for Z 55, A1 = 1000 Ci (37 TBq); and
        for Z > 55, A1 = 200 Ci (7.4 TBq)
        where Z is the atomic number of the nuclide.
      • (3) For beta emitters, A1 is determined by the maximum beta energy (E max) according to Table 2; and
      • (4) For alpha emitters, A1 is determined by the expression:
        A1 = 1000 A3
        where A3 is the value listed in Table 3;
    • (b) A2 is the more restrictive of the following two values:
      • (1) The corresponding A1; and
      • (2) the value A3 obtained from Table 3.
  • 3. For any single radionuclide whose identity is unknown, the value of A1 is taken to the 2 Ci (74 MBq) and the value of A2 is taken to be 0.002 Ci (74 MBq). However, if the atomic number of the radionuclide is known to be less than 82, the value of A1 is taken to be 10 Ci (370 GBq) and the value of A2 is taken to be 0.4 Ci (14.8 GBq).

II. Mixtures of Radionuclides, Including Radioactive Decay Chains.

  • 1. For mixed fission products, the activity limit may be assumed if a detailed analysis of the mixture is not carried out,
    A1 = 10 Ci (370 GBq)
    A2 = 0.4 Ci (14.8 GBq)
  • 2. A single radioactive decay chain is considered to be a single radionuclide when the radionuclides are present in their naturally occurring proportions and no daughter nuclide has a half-life either longer than 10 days or longer than that of the parent nuclide. The activity to be taken into account and the A1 of A2 value from Table 1 to be applied are those corresponding to the parent nuclide of that chain. When calculating A1 or A2 values, radiation emitted by daughters must be considered. However, in the case of radioactive decay chains in which any daughter nuclide has a half-life either longer than 10 days or greater than that of the parent nuclide, the parent and daughter nuclides are considered to be mixtures of different nuclides.
  • 3. In the case of a mixture of different radionuclides, where the identity and activity of each radionuclide are known, the permissible activity of each radionuclide R1, R2 . . .Rn is such that F1+ F2 + . . . Fn is not greater than unity, where:
    equation1
    Equation2
    Equation3

    Ai(R1, R2 . . .Rn) is the value of A1 or A2 as appropriate for the nuclide R1, R2. . . Rn.

  • 4. When the identity of each radionuclide is known but the individual activities of some of the radionuclides are not known, the formula given in paragraph 3. is applied to establish the values of A1 or A2 as appropriate. All the radionuclides whose individual activities are not known (their total activity will however, be known) are classed in a single group and the most restrictive value of A1 and A2 applicable to any one of them is used as the value of A1 or A2 in the denominator of the fraction.
  • 5. Where the identity of each radionuclide is known but the individual activity of none of the radionuclides is known, the most restrictive value of A1 or A2 applicable to any one of the radionucides present is adopted as the applicable value.
  • 6. When the identity of none of the nuclides is known, the value of A1 is taken to be 2 Ci (74 GBq) and the value of A2 is taken to be 0.002 Ci (74 MBq). However, if alpha emitters are known to be absent, the value of A2 is taken to be 0.4 Ci (14.8 GBq).