Low-Background Activation Analysis

NAA for ultrapure materials analysis

Richard M. LindstromAnalytical Chemistry Division

National Institute of Standards and TechnologyGaithersburg, Maryland

Detection Limit for Assay

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

where A =1/desired precision and b the peak integration width(Cooper 1970)

€

D(E) =A

tε (E)Γ(E)2bR(E)Bn (E) +

A2

4

⎡

⎣ ⎢ ⎤

⎦ ⎥

1/ 2

+A2

⎧ ⎨ ⎩

⎫ ⎬ ⎭

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), 239-

246.

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 -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, MonacoLNGS, 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)