Low-Background
Activation Analysis
NAA for ultrapure materials analysis
Richard M. Lindstrom
Analytical Chemistry Division
National Institute of Standards and Technology
Gaithersburg, Maryland
Detection Limit for g Assay
2 1/ 2


A
A 
A 
D(E) 
 
 2bR(E)Bn (E) 
t (E)(E) 
4 
2 


where A =1/desired precision and b the peak integration width
(Cooper 1970)
Detection limit is inversely proportional to the
detector efficiency and counting time, and
~proportional to the square root of the resolution
R and the environmental background Bn
Example: Composition of
Interplanetary Dust Particles
• Desired a factor ~106 improvement in sensitivity
over "normal” INAA
• 200x by longer, higher-flux irradiation
• 100x by increasing counting efficiency and time
• 7x by reduced background
– D. J. Lindstrom, Analysis of Submicrogram Samples by INAA, Nucl.
Instrum. Methods A299 (1990), 584-588.
Background: NIST vs. RCL
• Nearly all background at NIST (ground
level) is from cosmic muons
• Going 60 ft underground at NASA-JSC
reduced all cosmic components by a
factor 5
– R. M. Lindstrom, D. J. Lindstrom, L. A. Slaback, and J.
K. Langland, A Low-Background Gamma Ray Assay
Laboratory for Activation Analysis, Nucl. Instrum.
Methods A299 (1990), 425-429.
Example: Forensics
• Locating and measuring 100 fg Ir particle in 60 mg
of rock by INAA
– B. C. Schuraytz, D. J. Lindstrom, L. E. Marín, R. R. Martinez, D. W.
Mittlefehldt, V. L. Sharpton, and S. J. Wentworth, Iridium Metal in
Chicxulub Impact Melt: Forensic Chemistry on the K-T Smoking Gun,
Science 271 (1996), 1573-1576.
Other Counting
• LDEF:12 nuclides measured in stainless steel
• Genesis: 3 years of solar wind at L1
• Ambient 85Kr in 1.5 liters of air
– L. A. Currie and G. A. Klouda, Detection and Quantification Capabilities for
85Kr with the NIST Low-Level Gas Counting System: Impacts of Instrumental
and Environmental Backgrounds, J. Radioanal. Nucl. Chem. 248 (2001), 239246.
Lab Design 1
• Depth
• Floor space
• Lifting equipment
• Environment conditioning
–
–
–
–
Temperature
Humidity
Particulates
Radon?
Lab Design 2
• Utilities
– UPS, filtered power
– Network
– LN2
• Sample prep clean area (above ground?)
• Change room
• Chemical fume hood
• Assembly shop
– Zone refining & crystal growing?
Graded Shielding
• Room
• 50-100 cm salt or ultrabasic rock; 2 m water
• Several Ge detectors, separated by ~ meters
• Lead or iron shield
• Inner shield (old Pb, Cu, Fe)
Intermediate Depth Laboratory
• For materials characterization
• For equipment testing
• More accessible than Homestake
– NBSR: “America’s favorite neutron source.”
• Modest depth
– 30 m rock (70 mwe) attenuates 30x
Example: Cosmic-ray Neutron
Activation
• Activation at ground level
– Elements W, Au, Ta, In, Re, Sm, Dy, Mn
• Counting at 500 mwe in HADES
– M. J. Martínez Canet, M. Hult, M. Köhler, and P. N. Johnston,
Measurement of activation induced by environmental neutrons using ultra
low-level g-ray spectrometry, Appl. Radiat. Isotop. 52 (2000), 711-716.
CELLAR: Collaboration of
European Low-level underground
LAboRatories
EC-JRC-IRMM (HADES), Belgium (~225 m)
IAEA-MEL, Monaco
LNGS, Italy (~1700 m)
LNSCE, France (~2200 m)
MPI Heidelberg, Germany (~10 m)
PTB, Gemany (~925 m)
University of Iceland (~165 m)
VKTA (Felsenkeller), Germany (~50 m)