A spreadsheet for a Geiger counter has two columns, "count (60 s run)" and "net rate / "

PHYSICS · Multi-part · Fermi University Physics · Scientific Practice & Experimental Methods · Uncertainty in measurement · Designing and running experiments

Measurement and Method, Ch Running it: troubleshooting, the systematics hunt, safety and the record

Problem

A spreadsheet for a Geiger counter has two columns, "count (60 s run)" and "net rate /

\mathrm{s^{-1}}

", the second already typed in as a fixed number. A footnote reads: "a background of

0.22\,\mathrm{s^{-1}}

, measured earlier that day, has been subtracted." No background run's count or duration is given anywhere, and there is no column for the uncertainty of the net rate. ((a)) State what must be added to this record for the net rate to satisfy the rule that every net rate states the background subtracted and the uncertainty that subtraction added. ((b)) If the source run gives

n_{\rm s}

counts in a time

t_{\rm s}

and the background run gives

n_{\rm b}

counts in a time

t_{\rm b}

, write down

u(R)

for the net rate

R=n_{\rm s}/t_{\rm s}-n_{\rm b}/t_{\rm b}

, and say which two quantities are missing from this record that stop

u(R)

from being computed. ((c)) The counting chain's known input is a sealed caesium-137 check source, handled on an open bench. Write a five-line safety assessment for handling it: hazard, who is exposed, control, residual risk, emergency action.

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