An engineer measures a source with a large coaxial germanium detector
PHYSICS · Multi-part · Fermi University Physics · Nuclear Physics · Electronics, Instrumentation & Communication · Radiation interactions, detection and dosimetry
The Nucleus Resolved, Ch From a pulse to a number: detectors, resolution and what a spectrum establishes
Problem
An engineer measures a 60Co source with a large coaxial germanium detector. At a triangular shaper peaking time of τp=2μs the 1332keV full-energy peak is recorded centred at 1315keV. Raising the peaking time to τp=6μs, with everything else unchanged, moves the centroid up to 1327keV. She argues: "This proves charge trapping. Trapping reduces the total charge collected, and a longer shaping time lets the amplifier integrate more of that reduced charge before it decays away, which is why the peak moves up."
((a)) Using the ballistic-deficit fraction Tc/(4τp) for a triangular shaper, find the effective charge-collection time Tc implied by the 2μs measurement.
((b)) Predict the centroid expected at τp=6μs if the loss is really ballistic deficit, and compare with the 1327keV observed.
((c)) Explain the error in the engineer's reasoning and give the correct diagnosis.