Sunday, 5 April 2015

Packard 1000 TR Liquid Scintillation Analyser - Part 2

Continuing the teardown of this Liquid Scintillation Analyzer

Before disassembly i wanted to discover how the photomultiplier power supply operated. On investigation it was clear it operated quite simply. With three connections to the main control electronics it uses a simple +5v signal to enable each of the two outputs. This means i can re-use the power supply and photomultipliers in future projects.

Photomultiplier Power Supply

Then came removal of the motor control board, which is a simple drive control to operate two 115v AC motors that operate the top light cap and the sample load/unload mechanism.

Sample loading head assembly.
The sample loading assembly is a moderately simple affair. A sliding top allows the insertion of two sizes of sample vial. The location of this slider is indicated to the electronics with two optical sensors indicating if it's in either position.

Below this is a heavy metal bracket that holds an ionizer assembly and an arm that covers the top of the sample chamber. This is operated by one of the motors with two optical sensors and serves to exclude light from the sample chamber with some plush velvet material to aid light blocking.

The ionizer assembly is a heavy brass construction with five electrodes spaced evenly around where the sample vial enters the sample chamber. The ionizer i believe is to remove any static charge on the sample vial from handling. The ionizer power supply is a Chapman brand with an output of 5000v.

Ionizer assembly & electrodes.

The photomultipliers were removed and saved for a future teardown.

After the photomultiplier and sample chamber assembly was removed it was time to find the Barium 133 check source located inside the machine. After some fiddling with the sample load/unload mechanism and removal of the sample loading assembly i was able to see the source. A small metalic object about 4mm in diameter attached to the coiled spring that allows the load/unload mechanism to push the source into the sample chamber.

Barium 133 Source
The Barium source was dated 11 January 1996 with a 18.8 micro Curie activity. Barium 133 has a half life of 10.51 years so as of April 2015 it's current activity is now around 5.5 micro Curies. Measuring this using my Ludlum Model 3 and a SBT-11A Geiger Muller tube reads about 13,000CPM.

Barium 133 Source