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Lifetime of Excited Nuclear States
Victor Weisskopf
Lifetime of Excited Nuclear States.
The lifetime of an excited nuclear state (level) is along with its energy, spin and parity an important and characteristic propertyrequired for nuclear model description. This experiment which makes use of the so-called delayed coincidence method is well suited for the milli- to nano-second time range. Two γ transitions to and from an excited nuclear level provide the start and stop signal to measure the individual decay time of that level. Taking the decay time of many nuclei gives the whole decay curve in a major dynamic range to determine the lifetime with precision. For example with a dynamic range of about 1000:1 one obtains 8 half-lives. In this experiment the decays of 57Co and 44Ti are used to determine the lifetime of a level in 57Fe and 44Sc.
Weißkopf model for half-lives of γ-transitions in nuclei
γ- Energy [MeV] γ- Energy [MeV]
Hal
f-life
[s]
Hal
f -life
[s]
Bench
PM PM
HVCanberra
3002 D
HVCanberra
3002
PreampTennelecTC 145
PreampOrtec 113
Delay Line Amp
Ortec 460
Delay Line Amp
Ortec 460
TSCAOrtec 455
TSCAOrtec 455Start Stop
TACOrtec 467
ADC + MCACanberra
Multiport II
ComputerDell PC
Lifetime of Excited Nuclear States : Required Knowledge
Heisenberg uncertainty principle
Principles of nuclear level schemes
Principles of γ-spectroscopy, physics, laws and rules
Lifetime of a nuclear state, partial lifetimes
Weißkopf model of γ-decayShort time measurement methods, overview
Special: Method of delayed coin-cidences
Method of Time-to-Amplitude Con-verter
Fast-Slow-method
Function of NaI(Tl)-detectors
Modular nuclear electronics:
Pre-amplifier, Main Amplifier,
Timing single channel analyzer,
TAC (=Time-to-Amplitude-Con-
verter), MCA (= Multichannel-
Analyzer)
Types of signals
Time Calibrator, time resolution
Energy calibration, energy reso-
lution
Lifetime of Excited Nuclear States : Tasks and Goals
Set-up of detectors and HV (high voltage): some detectors require pos. ~800 -- 1000 Volts (slow multipliers, here 800 V), others require neg. ~1800 – 2000 Volts (fast multi-pliers, here -1850 V)Check signals after pre-amp and amps choosing one of the two available sources for lifetime measurement 57Co and 44Ti; relevant γ-lines should have an amplitude of ~ 1—2 Volts, not moreSet the single channel correctly on the relevant γ-line
Start time measurement in appro-priate range (4 or 8 μs), adjust count-rate by choosing proper detector-source distance, when o.k. startlonger run (overnight)Calibrate TAC with time calibrator(about 10 lines should fill the MCA-range)Evaluate lifetime from both measure-ments with errorGet energy spectrum of the sourceand some appropriate other calibra-tion lines Determine the overall time resolutionof the chosen set-up
Lifetime of Excited Nuclear States : WARNINGS
Caution with high voltage, get the right HV-sign for the detectors
Order switching on: First NIM-crate and power for pre-amps, second HV with appropriate sign and value
Order switching off: First HV off, then NIM-power off
Energy Eγ [keV]
Num
ber o
f cou
nts/
chan
nel
57Co spectrum taken with a Ge detector
Energy Eγ [keV]
Time [ns]
Time [ns]
Energy Eγ [keV]
Energy Eγ [keV]
Energy Eγ [keV]
Puls
es p
er c
hann
el
Time [ns]
Puls
es p
er c
hann
el
