health and safety laboratory - Digital Collections

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to a 1-1/4" diameter plastic counting planchet. Each planchet has been labeled
f
The samples are ashed at 500 to 550° C.in electrically-heated muffle
furnaces which hold individual racks. The ashing process is accelerated by
feeding in about one liter of oxygen per minute to each furnace and requires
slightly under one hour for completion* After cooling , the ash is transferred
with a gummed label which is a duplicate of the one used on the data card. This
maintains the proper continuity and agreement of serial numbers. The planchets
are then sealed between two layers of vinyl tape in groups of eighty. The
sealing is done with a high frequency heater which makes a ring seal around
M each planchet. The samples are spaced about 9" on centers and the resultant
tape can be readily handled in an automatic counter.
Jf Each tape contains four blanks consisting of unexposed papers run
_ through the process and four counter standards. Counter background is de-
"* termined from the blanks and counter efficiency and reliability is determined
9 from the standards.
Counting
w The samples are analyzed on automatic beta counters with Geiger tube
_ detectors . A reel of eighty planchets is loaded into the counter and the first
'• sample is positioned above the Geiger tube which is mounted face up. The
• sample is separated from the tube face by a single thickness of vinyl tape
and is held in position by a fixed stop which engages the side of the plan-
• chet. The counts are registered on a four Dekatron sealer. Simultaneously,
600 pulses per minute are applied to this sealer and are recorded on a dup-
•I licate parallel sealer. When 10,000 count plus time pulses are recorded on
if the first sealer, the number of time pulses is punched on paper tape by a
Data Transfer and Recording (DTR) system developed by the Instrumentation
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