777

Description

The Mirion 777 system is an ultra low-background lead shield for germanium gamma-ray detectors. It features 50% greater lead thickness than that of typical lead shields and is constructed from materials carefully selected for low background. The 777 shield is available in three versions which complement Mirion ultra low-background cryostats. The 777 shield can accommodate either a vertical dipstick slimline cryostat or a U-style cryostat. The 777A shield accommodates vertical dipstick cryostats only such as the 7500SL with a remote detector chamber (Model RDC-6). The 777B shield accommodates only U-style cryostats. All versions of the 777 system 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 shield 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 Mirion 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.