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Features

• 15 cm (6 in.) lead thickness
• Ultra low-background materials
• Versions for vertical or U-style cryostats
• Purge port for radon expulsion
• Graded tin-copper liner

Description

The CANBERRA 777 is an ultra low-background lead shield for germanium gamma-ray detectors. It features 50% greater lead thickness than typical lead shields and is constructed from materials carefully selected for low background. The 777 is available in three versions which complement CANBERRA ultra low-background cryostats. The 777 (pictured above) can accommodate either a vertical dipstick slimline cryostat or a U-style cryostat. The 777A accommodates vertical dipstick cryostats only such as the 7500SL with a remote detector chamber (Model RDC-6). The 777B accommodates only U-style cryostats, such as the 7915-30. All versions of the 777 are equipped with a swing-top door for sample chamber access and a door lift mechanism which allows the door to fit tightly against the shield body.

Background radiation from lead is dominated by ²¹⁰Pb because other elements are chemically separated fairly effectively in the refining process. ²¹⁰Pb decays with the emission of a 46.5 keV gamma ray. This gamma ray is generally absorbed by the tin-copper liner and may be below the energy range of the detector so it is of little concern. The ²¹⁰Bi daughter of ²¹⁰Pb, however, is of concern. It is an energetic beta emitter and bremsstrahlung from this beta gives rise to a continuum of background counts ranging up to 500 keV. For this reason, the 777 is constructed with layered bulk lead. The outer five inches is ordinary low-background lead while the innermost one inch is selected for ²¹⁰Pb content of about 20 Bq/kg. This thickness of lead will stop the bremsstrahlung from the outer lead volume.

The 777 has a graded liner to stop the lead K-shell x rays in the energy range of 75-85 keV. Liner materials are low background tin with a thickness of 1 mm (0.040 in.) and high purity copper with a thickness of 1.5 mm (0.060 in.).

The 777 is equipped with a gas port for the introduction of gas (bottled air or nitrogen) to flush the shield interior and reduce background from radon and radon daughters. Boil-off nitrogen from the LN₂ cryostat is a free source of gas but care must be taken to prevent liquid nitrogen from entering the shield and freezing the cryostat seals. An N₂ purge kit is available from CANBERRA for most cryostat types.

The 777A (see illustration in PDF) is supplied with a table which supports the shield above the LN₂ Dewar. This shield has an opening in the floor and a split annular back shield which surrounds the cryostat RDC neck. The 777B (see illustration in PDF) is attached to a steel plate which normally sits on the floor. The 777B has a side port which accommodates the horizontal arm of a U-style cryostat and a rectangular plug which is withdrawn vertically to provide access to the port. An optional cryostat stand (Model 777-5) is available to support the cryostat.