Activation Decay Times

The beam used for experimentation at the NSRL facility will result in activation of material exposed to it. All materials irradiated in the NSRL target room are to be controlled as radioactive until surveyed and released by a BNL Radiological Controls Technician (RCT). Samples that have had radioactive tracers such as ³H or ¹⁴C added to them shall be controlled as radioactive through out the entire process and will not be released as non-radioactive. When liquid samples are activated, there exists a potential for dispersion of radioactive material through spilling of the sample during handling or manipulation creating a contamination area. Activated samples containing liquids that are manipulated required additional Radiological Training (Benchtop/Dispersable Training), a designated (Posted) radiological area to perform work in, and a Radiation Work Permit (RWP). When samples have had enough time to decay they again become non-dispersible and no special radiological handling is required. Correct and appropriate biological handling techniques always apply regardless of the sample’s radiological status.

Below are tables indicating the type of beam delivered to the sample (proton or ion), absorbed dose delivered, and the required decay time for a sample to be considered non-dispersible. These tables must be considered when planning work to determine if your samples will be considered to be radioactive dispersible, which would require additional training and special handling. Work on the samples can proceed by workers with the appropriate training.  If work on the samples is planned, notification should be given to either the Medical Department or the Biology Department, as appropriate.

If activated samples need to come back to Medical, users are asked to let the NSRL Medical Liaison Scientist (631-344-2913) know well in advance so the Facility Support Division can have hoods/incubators posted and Radiological Work Permits written as appropriate.  Teams are urged to have at least 2 team members with Dispersibles Training in case of an unexpected emergency.  An HP Tech at NSRL will still need to survey materials and place a RAM tag on the samples.  Transportation arrangements can be made for users to get from NSRL to Medical with activated samples.  Users are authorized to transport properly contained activated samples in a government vehicle if they have a valid guest appointment and a valid driver’s license.  The Long Term Support Facility (LTSF) staff in Medical is also available for transportation needs.  Please advise the NSRL Medical Liaison of your plans ahead of time (i.e. before the run starts) so proper arrangements can be made.

Human cells must have “close proximity” double containment during irradiations at NSRL unless they are listed as Biosafety Level One by a vendor or cell repository.  The most convenient method for this containment has been to place the entire holder in a zip-lock baggy.  The team from the Biology Department will provide a variety of different sized bags at NSRL. 

The activation decay times are presented as graphs, in tabular form, with a fuller description following.

Tables exist for the following ions: Hydrogen (protons), Carbon, Oxygen, Silicon, Titanium, and Iron.

If exposure times are long, i.e. are no longer negligible compared to the decay times of the relevant isotopes (20 minutes for ¹¹C), then the tables below are an overestimate of the activation decay times, and users may want to make use of a dynamic decay time calculator that assumes a linear activation profile convoluted with the exponential decays of the activation products.  This calculator can be accessed here.  Contact the NSRL Liaison Physicist (631-344-3072 or 631-344-5830) or the RCT if you require assistance in using this application.

Figure 1: Activation Decay Times for Exposures of 0-10 Gray total.  Horizontal axis is logarithmic.

Figure 2: Activation Decay Times for Exposures of 0-10 Gray total.  Horizontal axis is linear.

Figure 3: Activation Decay Times for Exposures of 0-1 Gray total.

Here is the same information presented in tabular format.

Protons delivered to sample

Carbon ions Delivered to Sample

Oxygen ions Delivered to Sample

Silicon ions Delivered to Sample

Titanium ions Delivered to Sample

Iron ions Delivered to Sample

Radioactive Decay of Activated Dispersible Materials at the NSRL

1. General

We have to comply with Suffolk County, DOE and BNL limits for concentrations of radioactivity that could be released into the environment or spilled.  DOE specified activity (DPM) per area, calculated to be from a spill that is 0.5 cm deep.

•  These limits are fixed values of concentration.  Total radioactivity and total exposure play no role in complying with these particular rules.  Loss of control is the concern.  Derived concentration limits are very low because they are derived assuming constant continuous exposure for one year.  Additionally, Suffolk County wants all water on or in soil, even surface water from a spill, to be less than the Drinking Water Standards, which is also derived by assuming one year exposure.  Since NSRL activity concentrations exist briefly, there is a considerable margin of safety.  However, surface contamination above these limits after an accidental spill is still viewed as a noncompliance by regulators and DOE.
•  The NSRL beam activates materials, producing fixed proportions of radionuclides with different half-lives.  The isotopes of greatest concern are ⁷Be and ¹¹C, produced through spallation of target material by the beam.  ⁷Be has a half-life of 53 days, providing an effectively constant activity level over time periods of interest for NSRL.  ¹¹C decays with a half-life of 20 minutes.  All other isotopes have much shorter half lives and have decayed before there is access to the target room.
• Some have very long half-lives (years or days), and can thus be considered to remain at constant levels during the time of an experiment.  (Called R2 and R3)
   

Figure 1 shows the activity of these types of radionuclides as a function of time. R₁ (¹¹C) has a short half life, and over the period of 240 minutes has decayed substantially.  R₂ (⁷Be) and R₃ have long half-lives, and their activities remain essentially constant during this time.

2. Multiple Radionuclide Decay

But we actually measure the sum of these disintegrations (DPM per unit volume), and their sum is the property that must be less than Suffolk County/DOE/BNL limits.  The sum of these disintegrations per volume is shown in Figure 2

Figure 2. Sum of decay of three radionuclides, R₁ (short half-life),and two longer-lived species, R₂ and R_3., shown as the black circles.  The red line is the line for single isotope decay of R_1.  The net effect of the presence of the longer-lived species is to increase the time required for the mixed isotope sample to decay to the release limit, as shown.

3. Concentration effect.

·        The release limits (Suffolk County/DOE/BNL) are based on concentration.

·        If sample #1 of 1 milliliter is irradiated with 10 Gy of an ion from the NSRL beam, it is activated to 10Y DPM/1 ml, and we must wait N minutes (from the Table as calculated for that ion) for it to decay to meet the release limit.

·        If we irradiated a similar 1 ml sample with 1 Gy, it would be activated to 1 Y DPM per 1 ml; if we had irradiated another 1 ml sample with 0.1 Gy, it would be activated to 0.1 Y per 1 ml.

4. Post Irradiation Sample Processing Effects

·        Some experimental protocols require the use of the samples and solutions in the same conditions in which they were irradiated. In this case, the considerations in Paragraph 3 and times in the Table pertain directly.

·        Other experimental protocols require the dilution of the irradiated samples/solutions with fresh, unirradiated, non-radioactive solution.  Here are two examples:

o        If sample #1 (1 ml, irradiated with 10 Gy, activated to 10Y DPM/ml), is diluted to a final volume of 10 ml with fresh, unirradiated, non-radioactive solution, it will contain Y DPM/ml.  We see that this corresponds to the activity level induced by 1 Gy, and we can use the times corresponding to 1 Gy in the Table.

o       Likewise, if sample #1 is diluted to a final volume (solution as in c) of 100 ml, it will contain 0.1 Y DPM/ml, corresponding to the activity level induced by 0.1 Gy, and we can use the times corresponding to 0.1 Gy in the Table.

Training Requirements
 

• CA Radiobiology UserTraining (AD-USER-RADBIO) is required for all Users at NSRL.
• Radiation Worker 1 (RW-1) is required for entry into the NSRL target area
• Benchtop/ Dispersibles  Practical Skills Evaluation (AD-RWT-NSRL-P) is required for the manipulation of irradiated samples identified as radioactive dispersible material. This training, which is a short version of the BNL

Benchtop/ Dispersible course, and is specific for NSRL Users. It is only applicable for using dispersible materials in NSRL (building 958), Medical Department (building 490) in specific areas, and the AGS primary/secondary experimental areas used by NASA (building 912).  This includes but is not limited to addition or removal of material, liquids/media from sample containers, dissection or any other post mortem procedure, which could release fluids or material. Work with dispersibles requires following the requirements specified in a job specific Radiation Work Permit (RWP) and requires RCT coverage.

Note: opening the container top to allow CO₂ to enter the sample container while in the incubator does not constitute work with dispersibles.

Survey Requirements
 

• All samples removed from the target room shall be surveyed for radioactivity after irradiation.
• A BNL RCT shall conduct all surveys.
• Samples identified as radioactive shall be tagged or placed in a posted Radioactive Material Area (RMA).
• Samples that need to be removed from the building (NSRL) shall be surveyed as non-radioactive or be tagged as Radioactive Material (RAM) by an RCT prior to leaving the building.
• Radioactive materials shall have secondary containment for transportation.
• All Radioactive Material shall be transported, on-site, in a government vehicle.

Last Modified: December 28, 2009
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