(216f) Investigation of Emulsion Characteristics by Neutron Activation

Investigation of Emulsion Characteristics by Neutron Activation

Introduction

           Understanding the effects of a Radiological Dispersal Device (RDD), or ?dirty bomb,? has led to several engineering assessments.  One approach for mitigation of effects of a ?dirty bomb? explosion is to cover the affected area by an emulsion that prevents radioactive materials from migrating.  Laboratory and field tests are not amenable to the use of those radionuclides normally associated with an RDD, since it is undesirable to spread radiation sources over previously uncontaminated areas.  Non-radioactive materials of a similar chemical form can sometimes be used in both field and laboratory studies to provide an understanding of physical transport processes.  On the other hand, if knowledge of environmental transport is desired, detectible radiation emitted in radioactive decay can be used directly to determine distribution of materials.  One method for using the convenient property of detectibility is neutron activation. Some stable nuclei are good absorbers of neutrons and yield an activation product of short half-life that can be identified through conventional spectrometry.  An inert material, such as a dust sample taken during a field test, can be placed in a neutron field and activated. 

Isotopes of interest are ¹³⁷Cs and ⁹⁰Sr.  Use of these isotopes in field testing would create an extensive problem of remediation.  In both cases, a benign material can be used for a test, and the material subsequently irradiated to produce a radioactive material having a half-life of less than one day.  The stable isotope of cesium can be transformed to a metastable isomer ^134mCs, which emits a photon of energy 127.5 keV in 13 % of its disintegrations.  On the other hand, an interrogating radiation source, ²⁴Na, can be produced by neutron absorption in NaCl.  ²⁴Na can be a surrogate for a study of the effects of ⁹⁰Sr and its offspring ⁹⁰Y, which emit only β-particles.

           Laboratory measurements using CsCl and NaCl in a subcritical assembly have provided data to indicate that irradiation producing low-activity samples can be successfully used to investigate some of the important questions surrounding transport of radioactive materials in an explosion as well as the efficacy of protective measures.

Application?Secondary Radiation Production

           Charged particles passing near a nucleus undergo a change in direction and therefore a change in velocity.  This process is called bremsstrahlung, which means ?braking radiation,? and results in emission of photons.  Bremsstrahlung photons exhibit a continuous energy distribution.  The upper limit of the distribution is equal to the energy of the charged particle, while the minimum energy is zero. 

           For a β-particle, the bremsstrahlung contribution is described by the following equation (1):

where            f is the fraction of incident β-particle energy converted into photons,            Z is the atomic number of the absorber, and            E_max is the maximum β-particle energy, in MeV.

           Emulsions will likely be characterized by water as the solvent.  Therefore, a Z of 8 will be appropriate for bremsstrahlung energy estimation.  For ⁹⁰Sr as the damaging material in an RDD, ⁹⁰Sr will be in equilibrium with ⁹⁰Y.  Maximum β-particle energies are 0.546 MeV for ⁹⁰Sr and 2.28 MeV for ⁹⁰Y.  The fraction of β-particle energy converted into photons is estimated as follows:

           ⁹⁰Sr energy conversion fraction f = 3.5 x 10^-4 x 8 x 0.546 = 1.53 x 10^{-3            90}Y energy conversion fraction f = 3.5 x 10^-4 x 8 x 2.28 = 6.38 x 10^-3.

In an emulsion where constituents are primarily hydrogen, carbon, and oxygen, the energy conversion is therefore less than 1 %.

Creation of Samples

           Na activates well and produces a β-particle of end-point energy 1392.91 keV in 99.944 % of its disintegrations.  Decay of ²⁴Na also produces g rays of energy 1368.633 keV in every transition and 2754.028 keV in 99.944 % of the transitions (2).  These gamma rays can be used as internal calibration standards.  Hence, table salt was used as the activation target.

The solubility of NaCl is reported to be 36 g/100 mL water (3).  If an emulsion is subsequently added to NaCl and the system is irradiated, the b-emitters should be distributed uniformly within the emulsion.  Induced activity of the sodium is inferred from the photopeak count of the 1368.633-keV photon.  The half-life of ²⁴Na is 14.9590 hours.  Therefore, an irradiation time of 3 half-lives, or approximately 45 hours, will provide near-maximum activation of the Na.  NaCl and emulsion were placed in a 20-mL vial for irradiation and measurement.

Procedure

           For counting, the vial is placed upright on the face of a shielded 3 x 3 NaI detector.  Photon spectra are obtained by the following procedure:

           1.  Place a measured quantity of NaCl into a 20-mL vial.

2.  Irradiate the vial and NaCl in the MSU Subcritical Assembly for at least three half-lives of ²⁴Na, or approximately two days.

3.  Remove the vial containing NaCl from the SCA and count for 12 hours to obtain a baseline spectrum.

4.  Return the vial containing NaCl to the SCA for another activation.

5.  Remove the vial containing NaCl from the SCA.  Add a known quantity of emulsion to the vial and shake.

6.  Perform a 12-hour count of the vial containing the activated salt and the emulsion.

7.  From the two spectra, determine

           a.  the net count under the 1369-keV peak, and

b.  the gross count in a region of interest selected to include photons from bremsstrahlung.

8.  Normalize the spectrum from the vial with emulsion to the spectrum from the same vial without emulsion.

9.  Subtract the gross bremsstrahlung region of interest (ROI) counts for the vial without emulsion from the corresponding bremsstrahlung ROI for the vial with emulsion.

Table 1 contains a summary of these results for the sixteen samples analyzed by this procedure.  Six different emulsions were provided for assessment, identified in column 1.  The LL series emulsions show a greater bremsstrahlung contribution than do the Socorro, the 9.6x, and the 10.6x emulsions.  In this table, only the 1369-keV photon peak was used for normalization.  The region of interest for bremsstrahlung radiation was arbitrarily chosen as the energy range 150-450 keV.
Table 1.  Bremsstrahlung Contribution.

Conclusions

           The significance of the method of taking difference between the ROI counts of two spectra can be analyzed by taking the difference between ROI counts of spectra for the samples containing NaCl only.  Two spectra from separate vials are normalized to the integral count of the 1369-keV photon from ²⁴Na disintegration.  The difference in counts from the bremsstrahlung ROI is computed.  This difference may be compared to the corresponding one calculated from the vial with emulsion and the one from the same vial without emulsion.  Gross areas of the bremsstrahlung region are used in the calculation.  The magnitude of fractional differences ranges from ~0 to 0.092, with an average fractional difference of 0.0425.  The LL, LL + PB, and LL + IX emulsions show a greater fractional difference than 0.0425, whereas the Socorro, 9.6x, and 10.6x emulsions have a smaller fractional difference. 

           Sensitivity analysis can be performed to determine the effect attributable to the definition of the region of interest for bremsstrahlung.  Also, analytical removal of the contribution of the annihilation peak at 511 keV may be performed.  However, this contribution is present in all sets of data, and differencing calculations effectively delete any effect attributable to annihilation counts.

REFERENCES

1.  Cember, Herman, Introduction to Health Physics, 2^nd Edition,
New York: Pergamon Press, 1988.

2.  Ekstrom, L. P. and R. B. Firestone, ?WWW Table of Radioactive Isotopes, database version 2/28/99 from URL http://ie.lbl.gov/toi/index.htm.?

3.  Dean, John A., Lange's Handbook of Chemistry, 15^th Edition,
New York:  McGraw-Hill, 1999.