Instruments - SHRIMP II

 

Following on from the success of SHRIMP I, SHRIMP II was the commercial prototype. SHRIMP II incorporated a number of changes from SHRIMP I in terms of the design and configuration.

Kohler illumination was configured as the standard mode of operation and with a stronger demagnification in the condensor lens (7 in SHRIMP II vs 4 in SHRIMP I) allowed a higher throughput of ions and therefore higher beam densities and primary beam strengths (factor of three).

The slit-einzel triplet system in the source chamber was replaced with a quadrupole triplet, which has lower aberrations allowing more useful beam through the source slit.

All power supplies and many apertures/slits were under computer control allowing the user to largely configure the instrument from the terminal.

SHRIMP II was designed with a single collector but with the intention of incorporating a multiple collector (for Pb and stable isotope analysis). The primary column was also reconfigured to allow a Cs metal gun and duoplasmatron to be mounted in a Y configuration with electrostatic switching between beams. Following several modifications, the multiple collector is now in operation. The Y column was dropped following the change to the Kimball Physics Cs zeolite gun, which was mounted on a flange to match the duoplasmatron mounting flange. As such there was no advantage in having both mounted at the same time.

SHRIMP II was designed with stable isotope analysis as a prime motivation. However, owing to a number of competing instrument developments (multiple collection, SHRIMP RG) the stable isotope work was initially only intermittently pursued. Stable isotopes are best analysed as negative ions with optimum ion yields from a Cs+ primary beam requiring a source and a polarity change from the configuration for positive ion analysis used in U-Pb.