Well Counters and Probes Instrumentation

Subgroup IV: Instrument Operations and Quality Control

uptake probe/well counter calibration frequency

daily before first use

uptake probe/well counter calibration purpose

to set the x-axis scale by tweaking the high voltage until the photopeak is centered in the pulse-height analyzer

What source is used to calibrate an uptake probe/well counter?

Cs-137 (662 keV, 10-200 uCi for probe, <1 uCi in well counter)

Why must we obtain background counts on a scintillation spectrometer?

to subtract from clinical/rad safety measurements

How much background is too much?

twice the normal background

What are some causes of high background?

contamination, patients, or other extraneous sources

How often must constancy be performed on an uptake probe/well counter?

daily

uptake probe/well counter constancy procedure

• reference source in standard geometric position

• record 1-min count

• verify that the energy spectrum looks normal, the photopeak is centered in the window

energy resolution definition

how well does the crystal differentiate between photon energies

What source is used for energy resolution of an uptake probe/well counter?

Cs-137 (662 keV)

Why do we use Cs-137 for energy resolution?

it is monoenergetic

How often is percent energy resolution performed on an uptake probe/well counter?

quarterly

percent energy resolution of uptake probe/well counter procedure

• determine the spectrum of the source

• calculate the Full Width Half Maximum

What is the Full Width Half Maximum used to measure?

the percent energy resolution

normal/expected values for percent energy resolution

8-12% (smaller number implies better energy resoluion)

causes for worsening of energy resolution

cracked crystal, uncoupling of crystal, crystal hydration

What should you do if the percent energy resolution of an uptake probe is calculated too high?

recalibrate, recheck, replace

well counter/uptake probe chi-square test purpose

determine count reproducibility

What source is used for a chi-square test on a well counter/uptake probe?

Cs-137 (662 keV)

well counter/uptake probe chi-square test procedure

10-20 1-minute counts, each more than 10,000 counts

What is a normal p value for a chi-square test?

0.1 < p < 0.9

What should you do if the chi-square test results of an uptake probe is calculated are outside the normal range?

recalibrate, recheck, replace

well counter/uptake probe efficiency factors purpose

to correlate counts measured with activity, in a quantitatively correct way

format for uptake probe/well counter efficiency factor

cpm/dpm

efficiency factors procedure

• decay-correct reference source is calculated for current day (give dpm)

• energy window should match the appropriate situation

• count source and subtract background (give cpm)

What is an acceptable value for efficiency factor?

<1, but not too much less than 1

causes of poor efficiency factor results in a well counter/uptake probe

calibration, peaking, geometry

What should you do if the efficiency factor of an uptake probe is calculated too low?

recalibrate, recheck, replace

Given this calibration data, what is the correct high voltage setting?

509

energy resolution formula

energy resolution = (FWHM (keV)) / (Energy (keV)) * 100%