Activation table of elements

Activation table of the elements ^a

An updated version of this table is in preparation. Any comments to Sax Mason

This table allows you to calculate the activation of a sample after it has been in a neutron beam for one day and the amount of time for it to decay to 2nCi/g or less, which is the limit for shipping a sample as "nonradioactive". It also displays the anticipated exposure you may receive when removing the sample from the instrument. A sample calculation is included at the end of the table. Storage time is the time required for a sample of the pure condensed-phase element exposed to a "standard" neutron beam to decay to 2nCi/g or less. Prompt activation gives the anticipated activation for the pure solid elements 2 min after the neutron exposure ceases. Contact dose is that expected from a 1g sample of the pure element from the prompt activation. Elements with a dash for the entries in all three columns do not show any activation. Those marked with a single asterisk are radioactive before exposure to the neutron beam; apart from Tc and Pm, they are all a-particle emitters. Bismuth is a special case; it is stable before exposure to the beam, but the activation product is an a-emitter.

^a These entries are derived by Mike Johnson of ISIS for decay times to 10⁵ and to 10⁴Bq/cm³ for 5-cm³ pure solid samples of the elements exposed to a neutron beam for 1 day at an intensity comparable to that on HIPD with LANSCE operating at 100µA. They are augmented by calculations from NIST of the activation from a 1-day exposure to a 10⁷n/s-cm² reactor thermal beam (marked †).

Using the following procedure and example, estimate the anticipated activation of a sample exposed to the neutron beam in a LANSCE instrument from the table. If you cannot estimate the activation, call your LANSCE contact.

Example: For 5g YBa₂Cu₃O₇ sample on NPD at 75mA for 24 hours (1 day).

1. Compute the mass fractions for all elements in the sample.

Example: The mass fractions are 0.13 Y, 0.4l Ba, 0.29 Cu, & 0.17 O

2. Obtain the prompt activation from the product of these mass fractions and the corresponding elemental activation values in the table. Multiply the contact dose by the sample mass that is exposed to the beam to obtain the expected contact dose from the entire sample.

Example continued:

prompt activity = 0.13x 1000 + 0.41x80 +0.29x 10000 + 0.17x0.0 = 3060nCi/g

contact dose = 5x(0.13x0.9 + 0.41x0.1 + 0.29x8.5 + 0.17x0.0) = 13.1mR/hr

3. Scale these activation values by the instrumental factor (below) and the beam current as a fraction of l00mA to obtain the estimated values for the sample and actual beam conditions.

Instrumental factors for LANSCE

HIPD 1.00 (reference) 
SCD 1.44 
FDS 0.50 
CQS 1.65 
NPD 0.025 
PHAROS 0.060 
LQD 0.033 
SPEAR 0.04 1

Example continued:

prompt activity = 3060x0.025x75/100= 57nCi/g

contact dose = 13.lx0.025x75/100 = 0.25mR/hr

4. To obtain the storage times, t, for each element in the sample, correct the appropriate times given in the table, t_s, by:

t = t_s(0.693+0.lxlog_eC),

where C is the product of the mass fraction, instrumental factor and beam intensity ratio to l00m A. This expression assumes that the storage times given in the table are roughly 10 half-lives for the slowest decaying isotope of significant concentration in the activated element The storage time for the sample is then the largest of the resulting times.

Example continued

Y storage time = 24d(0.693+0.lxlog_e(0.13x0.025x75/100)) = 2.2d

Ba storage time = 150h(0.693+0.lxlog_e(0.41x0.025x75/100)) = 1.3d or less

Cu storage time = 7.4d(0.693+0.lxlog_e(0.29x0.025x75/100)) = 1.3d

O storage time = 0 (no activation expected)

Thus the storage time for this sample of YBa2Cu₃O₇ is determined by the decay of the Y as 2.2 days. At that time, the sample should show residual activation of less than 2nCi/g.

These estimates are based on a neutron exposure time of 1 day. The actual level of activation can drastically change if the exposure is more or less than 1 day. Determining this change cannot be accomplished using these simplistic calculations. Therefore, we require that all samples (no matter how long the neutron exposure is) be surveyed before they leave the facility.

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