Development of a low background liquid scintillation counter for a shallow underground laboratory

Abstract

Pacific Northwest National Laboratory has recently opened a shallow underground laboratory intended for measurement of lowconcentration levels of radioactive isotopes in samples collected from the environment. The development of a low-background liquid scintillation counter is currently underway to further augment the measurement capabilities within this underground laboratory. Liquid scintillation counting is especially useful for measuring charged particle (e.g., β, α) emitting isotopes with no (or very weak) gamma-ray yields. The combination of high-efficiency detection of charged particle emission in a liquid scintillation cocktail coupled with the low-background environment of an appropriately-designed shield located in a clean underground laboratory provides the opportunity for increased-sensitivity measurements of a range of isotopes. To take advantage of the 35 meters-waterequivalent overburden of the underground laboratory, a series of simulations have evaluated the scintillation counter's shield design requirements to assess the possible background rate achievable. This report presents the design and background evaluation for a shallow underground, low background liquid scintillation counter design for sample measurements.

Liquid scintillation counting of environmental radionuclides: a review of the impact of background reduction

“…Although the ingrowth of daughters may need to be considered for correcting the observed activity to that of 227 Ac, it can be shown that over a 2083 min count time, only two daughters will have sufficient activity to contribute more than 1 % of the 227 Ac activity. One of these ( 227 Th) can be removed by alpha/beta discrimination and More detail on design considerations for reducing the background count rate is provided in [8] J Radioanal Nucl Chem the other ( 223 Fr) can be spectrally resolved due to a large difference in end-point decay energy from 227 Ac (1099 keV and 45 keV, respectively). In conclusion, a very low background LSC system may provide a practical means to take advantage of 227 Ac as a chronometer of deep ocean transport processes.…”

“…More detail on design considerations for reducing the background count rate is provided in [8]. The values presented are based on seven replicate measurements and 21.5 % measurement efficiency a 1 TU (tritium unit) atomic ratio of tritium:hydrogen of 10 -18 , equivalent to 0.119 Bq/kg of water beads to increase the yield of combustion water, reduce the effect of color quenching, and reduce the quantity of sample needed to obtain sufficient analyte for a precision measurement.…”

“…We anticipate that an appropriately designed custom LSC system constructed of low background materials and placed in an underground laboratory could potentially achieve a background reduction on the order of 10-100 times the background rate seen in above ground commercially-available LSC systems. This level of background reduction is supported by early radiation transport simulations used to evaluate the proposed custom low background LSC system, and is reported elsewhere [8]. In this article the objective is to evaluate the utility of such a low background LSC system.…”

“…Background is one limitation of radiometric measurement sensitivity (others include sample size, counting efficiency, and count time). The development of low background liquid scintillation counters is described elsewhere [1,8], though it suffices to note the major steps in reducing the rate of unwanted background events include: (1) Passive shielding of the LSC vial and scintillation cocktail from radioactivity naturally found in the construction materials of the laboratory, typically achieved through use of lead to attenuate gamma rays from naturally occurring radioactive isotopes; (2) Dual or multiple photon detection units operated in coincidence to reduce the number of false events not attributed to real scintillation light produced within the sample vial; (3) Anti-coincidence detectors to veto (exclude) events generated in the LSC vial and scintillation cocktail due to the interaction of secondary cosmic ray induced particles (muons, protons, and neutrons); (4) A number of post-data collection techniques employing analysis energy windows, pulse-based a/b discrimination, and other features to effectively reduce the background rate of unwanted events in the region of interest for the scintillation measurement. While this list is presented in broadbrush and is explicitly not all-inclusive, items (1)- (3) are all amenable to individual improvement.…”

Assay methods for 238U, 232Th, and 210Pb in lead and calibration of 210Bi bremsstrahlung emission from lead

“…However, this lead was purchased in 2014 from Seafab Metals Company of Casa Grande, AZ. This lead was used in the construction of the middle layer of the shield for a low background liquid scintillation counter [16] located in the PNNL shallow underground laboratory [2] in Richland, WA.…”