Twenty Five Years of Neutron Activation Analysis: A ... · 186W(n, )187W 12.80 75As(n, )76As 13.56...
Transcript of Twenty Five Years of Neutron Activation Analysis: A ... · 186W(n, )187W 12.80 75As(n, )76As 13.56...
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Twenty Five Years of
Neutron Activation Analysis: A Personal Perspective on Utilization of the
Techniques
Dr. S. Landsberger, Coordinator
Nuclear and Radiation
Engineering Program
University of Texas at Austin
Nuclear Engineering Teaching Lab
Austin, Texas, USA
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NAA Facilities
• As of 2010, there are 236 research reactors worldwide and another 6 under construction.
• Neutron activation analysis (NAA) still remains the most used technique in these facilities.
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Interferences in NAA
• The use of thermal neutrons is the mainstay of NAA for all the facilities.
• The presence of aluminum, chlorine, sodium and manganese can inhibit the determination of several short-lived radionuclides due to increased backgrounds from the various reactions: 27Al(n,γ)28Al, 37Cl(n,γ)38Cl,
23Na(n,γ)24Na and 55Mn(n,γ)56Mn.
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Interferences in NAA
• For medium-lived NAA the presence of 23Na(n,γ)24Na and 81Br(n,γ)82Br can also greatly add to the Compton continuum to the point that elements such as arsenic need to be determined by radiochemistry.
• Reactions of 45Sc(n,γ)46Sc and 59Co(n,γ)60Co can also add greatly to the Compton continuum limiting the detection of other elements such as silicon, nickel, iodine, several rare earths and even zinc.
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NAA and
Resonance
Integrals
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Epithermal NAA Nuclear Reaction
59Co(n,)60mCo 1.91
186W(n,)187W 12.80
75As(n,)76As 13.56
109Ag(n,)110Ag 15.38
115In(n,)116mIn 16.33
81Br(n,)82Br 18.52
127I(n,)128I 23.71
121Sb(n,)122Sb 33.90
68Zn(n,)69mZn 43.06
124Sn(n,)125mSn 61.54
29Si(n,p)29Al -------
0
I
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3-L
RSR
Pneumatic
Tube
System
TRIGA Reactor
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Timers
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Compton Suppression
• Since early 1990’s Compton suppression neutron activation analysis (CSNAA) has been effectively employed to quite dramatically lower detection limits for many elements
• Best exploited when the gamma ray of analytical interest is the only or major one that is involved in the beta decay process
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Compton Suppression
• Many more institutions now have Compton suppression instrumentation in their laboratories
• Many more published papers on characterization, development and applications in NAA, environmental counting and fission product identification
• CSNAA can be judiciously used in biological specimens, but using ENAA can even further reduce backgrounds levels
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Compton Suppression
• Compton suppression is ideal for radionuclides that emit single gamma-rays or gamma-rays that are not in coincidence with other photons in the decay scheme.
• Some examples include 137Cs in environmental samples, 198Au in neutron irradiated geological samples and 203Hg and 128I in neutron irradiated biological samples.
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Examples of Some Prime Radionuclides for CSNAA
65Zn (1115 keV) 52V (1434 keV) 51Ti (320 keV) 80Br (618 keV)
203Hg (279 keV) 198Au (411 keV) 51Cr (320 keV)
115Cd/115In (336 keV) 128I (443 keV)
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Decay Scheme of Single 279 keV Emitting
Gamma Ray of the 203Hg Radionuclide
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Decay Scheme of Two Strongly Coincident Emitting 1368 and 2754 keV Gamma Rays of
the 24Na Radionuclide
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Major Interferences Giving Rise to High Backgrounds or Spectral Interferences
• 24Na - 1368, 2754 keV
• 28Al - 1779 keV single emitting photon
• 38Cl- 1642, 2167 keV
• 56Mn - 846, 1810, 2112 KeV
• 60Co - 1173, 1332 keV
• 46Sc - 889, 1120 keV
• 59Fe - 1098, 1291 keV
• 75Se on 203Hg
• 239U 239Np on 115Cd/115In
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Weakly Coincident Gamma Rays
• Some radionuclides that have two or more gamma rays in their decay can also benefit from CSNAA, if one of them has weaker coincidences with the remaining gamma ray(s).
• The radionuclides 76As (559 keV) and 122 Sb (564 keV) are two such cases in point
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NIST 1635 Coal Short-Lived NAA
1
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Cou
nts
per
ch
an
nel
Energy (keV)
A
B
C
D
A: Thermal, Normal
B: Epithermal, Normal
C: Thermal, Compton
D: Epithermal, Compton
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NIST 2711 Soil
0
500
1000
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2500
3000
3500
4000
4500
5000
Co
un
ts p
er
ch
an
ne
l
Energy (keV)
A
B
C
D Eu-152
Sc-46
Zn-65
A: Thermal, Normal
B: Epithermal, Normal
C: Thermal, Compton
D: Epithermal,
Compton
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NIST 2711 Soil
1
10
100
1000
10000
100000
Co
un
ts p
er
ch
an
ne
l
Energy (keV)
D
Co-58 Tb-160
A: Thermal, Normal
B: Epithermal, Normal
C: Thermal, Compton
D: Epithermal, Compton
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Fully Automated Fast Pneumatic System for NAA
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Fully Automated Fast Pneumatic System for NAA
• Analyze short-lived radionuclides
• Cyclic sample irradiations
• Minimize user interaction
- reduce exposure
• Provide an efficient means of analyzing multiple samples
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Considerations
Shielding
• Detector Setup
-Outer Shield: Lead
-Inner Shield I: Cadmium
-Inner Shield II: Copper
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Reactor
Core
CO2
Pump
2-Way
Loading
Hopper Sample
Injection/Ejection
Valve
Waste
Bin
Detector
Housing and
Sample Stop
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Setup
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Photon Attenuation
• Photon attenuation remains a problem that is constantly overlooked particularly when trying to correctly determine low energy gamma rays ordinary matrices.
• The problem is severely compounded when there samples have high-Z materials
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Neutron Flux Monitoring • Another area which needs attention is the
monitoring of neutron fluence that a sample receives for short-lived NAA.
• While some reactors such as the 20 KW Canadian SLOWPOKE and the 30 KW Chinese Miniature Neutron Source reactor (MNSR) have very stable neutron fluxes that vary only 1-2 %, other reactors such as TRIGA have neutron fluences that vary ± 5-7%.
•
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Rare-Earth Analysis
• Fission interferences
• Spectral interferences
• It still remains surprising how many paper using NAA for rare-earth analysis do not take into account any of these interferences
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Compton Suppression for Rare-Earth Analysis
1
10
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1000
10000
100000
Co
un
ts p
er
ch
an
nel
Energy (keV)
D
Tb-160
AB
C
A: Thermal, Normal
B: Epithermal, Normal
C: Thermal, Compton
D: Epithermal, Compton
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New Directions
• In the recent past my group has been involved in the nuclear forensics and nuclear fuel cycle experiments.
• We have judiciously used NAA to produce fission products to test our low level Compton suppression system
• To produce surrogates for nuclear fuel cycle separation experiments conducted at national laboratories
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Training and Teaching • One are that is neglected is the pedagogical aspects
of NAA.
• It has been 40 years that the last comprehensive NAA book has been written by Soete, Gijbels and Hoste (1972)
• While different aspects of NAA are covered in other nuclear and radiochemistry courses, there appears to be no course completely dedicated to NAA.
• Given the enormous advances that NAA has undergone, it behooves the community to write another book.
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SUMMARY AND CONCLUSIONS
• Considering the costs of reactor resources, it is very advantageous to install both epithermal and Compton suppression systems to augment the array of elements that can be better determined in a variety of samples.
• While a cyclic system may only be beneficial for a few elements, the automation and control of irradiation, decay and counting times for short-lived NAA would greatly enhance quality assurance procedures.