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 property
required 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
lifetime of a level in 57Fe and 44Sc.
44Ti
are used to determine the
Half-life [s]
Half-life [s]
Weißkopf model for half-lives of γ-transitions in nuclei
γ- Energy [MeV]
γ- Energy [MeV]
HV
Canberra
3002 D
HV
Canberra
3002
PM
PM
Bench
Preamp
Tennelec
TC 145
Delay Line
Amp
Ortec 460
Preamp
Ortec 113
TSCA
Ortec 455
Start
Stop
TAC
Ortec 467
ADC + MCA
Canberra
Multiport II
Computer
Dell PC
TSCA
Ortec 455
Delay Line
Amp
Ortec 460
Lifetime of Excited Nuclear States : Required Knowledge
¾ Heisenberg uncertainty principle
¾
Fast-Slow-method
¾ Principles
¾
Function of NaI(Tl)-detectors
¾
Modular nuclear electronics:
of
nuclear
level
schemes
¾ Principles
of
γ-spectroscopy,
Pre-amplifier, Main Amplifier,
physics, laws and rules
Timing single channel analyzer,
¾ Lifetime of a nuclear state, partial
TAC (=Time-to-Amplitude-Con-
lifetimes
verter), MCA (= Multichannel-
¾ Weißkopf model of γ-decay
¾ Short time measurement methods,
overview
¾ Special: Method of delayed coincidences
¾ Method of Time-to-Amplitude Converter
Analyzer)
¾
Types of signals
¾
Time Calibrator, time resolution
¾
Energy calibration, energy resolution
Lifetime of Excited Nuclear States : Tasks and Goals
¾ Set-up of detectors and HV (high
¾
Start time measurement in appro-
voltage): some detectors require
priate range (4 or 8 μs), adjust count-
pos. ~800 -- 1000 Volts (slow
rate by choosing proper detector-
multipliers, here 800 V), others
source distance, when o.k. start
require neg. ~1800 – 2000 Volts
longer run (overnight)
(fast multi-pliers, here -1850 V)
¾
¾ Check signals after pre-amp and
(about 10 lines should fill the MCA-
amps choosing one of the two
available
sources
measurement
57Co
for
lifetime
and
range)
¾
44Ti;
relevant γ-lines should have an
¾
Get energy spectrum of the source
and some appropriate other calibra-
more
on the relevant γ-line
Evaluate lifetime from both measurements with error
amplitude of ~ 1—2 Volts, not
¾ Set the single channel correctly
Calibrate TAC with time calibrator
tion lines
¾
Determine the overall time resolution
of the chosen set-up
Lifetime of Excited Nuclear States : WARNINGS
¾ Caution with high voltage, get the right HV-sign for the detectors
¾ Order switching on: First NIM-crate and power for pre-amps, second
HV with appropriate sign and value
¾ Order switching off: First HV off, then NIM-power off
Number of counts/channel
57Co
spectrum taken with a Ge detector
Energy Eγ [keV]
Energy Eγ [keV]
Time [ns]
Time [ns]
Energy Eγ [keV]
Energy Eγ [keV]
Energy Eγ [keV]
Pulses per channel
Time [ns]
Pulses per channel