REACTOR NEUTRON FLUX LEVEL BY FOIL MEASUREMENT

When a specimen comprised of a single isotopic species is irradiated in a nuclear reactor, the resultant activity at the end of the irradiation period is given by the expression:

Where:

• alpha_i = Activity of the sample in disintegrations/s (dps)
• t_i = Irradiation time t_i (sec)
• phi = Thermal neutron flux at the sample, neutrons/cm²/s
• V = Sample volume, cm³
• Sigma_A = Macroscopic neutron absorption cross-section of the sample for thermal neutrons at 2200 m/s, barns.
• lambda = Decay constant of the radioisotope produced by the absorption of neutrons, s^-1
• 1.128 = Correction for reactor spectrum of neutrons (presumed Maxwellian distribution)

Following the irradiation, the activity of the radioisotope will decay according to the following relationship:

Where:

• alpha = Sample activity, dps
• alpha_i = Sample activity at the end of irradiation, dps
• t_d = Time after irradiation: decay, or "cooling time", s

Note that at any time, the activity is represented as:

Thus, N, the number of radioactive atoms in the sample can be easily computed.

When the radioisotope is counted by a detector, the count recorded during an interval of time t_c is proportional to the activity, the count time (t_c), and the efficiency of the detector.

If C₁ counts are recorded at t₁, and C₂ recorded at t₂, then:

Where:

Taking the natural log of both sides of the expression, we obtain:

Which has the form y = B - Ax, and we realize that a plot of C vs. t would be linear on semi-log paper, provided:

• The source-detector distance and geometry remain constant during the measurements,
• Only one radioisotope contributes to the count, and
• Dead-time corrections for high count rates are applied, if required.

To determine the neutron flux to which a metallic foil has been exposed by calibration of the detector and analysis of the activation and resultant decay of the radioisotope produced.

A "short" half-life gamma-ray emitting sample will be irradiated in the reactor for use. Following the instructions of the reactor operator and/or the instructor:

1. Select a suitable metallic foil, clean its surfaces, and determine its mass.
2. Place a suitable metallic foil in small sample capsule.
3. Use tweezers to avoid contamination of the foil.
4. Place the capsule in a screw-top sample holder.
5. Consult the instructor and/or reactor operator to determine approximate foil activity desired.
6. Using laboratory software, determine irradiation time and power level for the sample.
7. Place the sample into the core, notifying the operator.
8. Record the clock time.
9. After the desired irradiation time, remove the sample holder from the core, notifying the operator.
10. Record the clock time.
11. Remove the sample holder from the pool, using proper handling techniques and monitoring procedures.
12. Remove the inner capsule.
13. Extract the sample foil from the capsule inner holder, using tweezers, and transfer the sample to the sample tray of the counter.
14. Monitor the transfer process.
15. Take successive one-minute counts of the sample, noting the starting (clock) time of each count (start on even-minute intervals on the same clock as used in part 1 and 2).
16. Plot cpm vs. clock time as you take data (semi-log plot).
17. Remove the foil and, using one of the laboratory (small) calibrated sources which has a similar energy gamma (or beta), take a count to establish detector efficiency.
18. Note the clock times of sample entry and removal from the core (from the operations log book).

1. Plot the data (cpm vs time) on semi-log paper.
2. Calculate the decay constant for the radioisotope.
3. Does the value agree with published data?
4. Does the data generate straight lines? If not, why not?
5. Calculate the detector efficiency from the lab standard source.
6. Use the data to solve the decay equation for the activity of the sample at the time of removal from the core.
7. Determine the effective thermal neutron flux to which the sample was exposed.