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The Underground Lab in LASR

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We are very fortunate to have access to the underground experimental area in the Laboratory for Astrophysics and Space Research (LASR), a unique space originally built and used by J. Simpson's and T. Turkevich's group. Six feet of concrete over this laboratory remove a large fraction of secondary cosmic rays, making it ideal for development and testing of low-background devices as well as for stocking of radio-pure materials, away from cosmogenic activation.

 

Figure 1. Figure 1.
Cross-section view of the LASR building, showing the underground laboratory and ~60 m.w.e. detector-testing station.

 

Figure 2. Figure 2.
Components of neutron flux as a function of underground depth. The measured overall neutron flux reduction in our lab is a factor of 10 with respect to ground level, and an additional factor of 10 at the bottom of the pit (rough spectral information has also been obtained using the Bonner spheres method).

 

Several tonnes of very low activity Pb shielding in ingot and brick form are available and have been recently measured at PNNL to contain less than 0.018 Bq/kg of Po210 (i.e., comparable to archaeological Roman lead). Some of this lead is already in use in the CAST experiment, in the SOLO counting station in the Soudan Underground Laboratory, and another fraction will be employed in MAJORANA. Two convenient counting stations for characterization of intrinsic radioactivity in detector materials are also in place, as well as a low-background ORTEC Ge diode and a number of NaI scintillators within individually-shielded counting stations. A crane-equipped well 3' in diameter and 53' deep, steel-lined to avoid Rn accumulation (a fresh-air level of < 0.3 pCi/l has been measured at the bottom) makes it possible to test detectors "in house" under an approximate 60 meters of water equivalent overburden. Two low background lead shields, designed to be lowered into the well, are also available. This represents an enormous advantage in reducing the time and costs spent in characterizing intrinsic detector backgrounds, eliminating travel to underground locations until a device is deemed ready for installation.

 

Figure 3. A view down the well during the descend of a Pb shield. Notice the steel lined walls, a measure against Rn accumulation.

 

In the particular case of detectors based on superheated liquids, where Pb shielding against gammas is superfluous, a well of this depth is already comparable to much deeper locations in terms of the neutron-induced background Figure 1. The whole laboratory area is a positive-pressure class 10,000 clean room. A 10'x10' soft-wall class 100 clean room has been added, a convenient asset for the assembly of low-background detectors.

 

Figure 4. Some 35 years ago, the steel pipe around our well was probably raising eyebrows of a few passersby.

 
 
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