Speed of Light
Experiment using positron annihilation and ultrafast timing techniques
Speed of light.
The speed of light c dominates most processes in the universe and
has outstanding importance for the principle of relativity.
The speed of light is nowadays fixed by definition to replace the
standard meter. In the present experiment the speed of light is
measured to study fast timing methods at the extremes. One makes
use of the simultaneous emission of two annihilation quanta from the
positronium decay and their strong angular correlation (180 degrees).
Two ultrafast plastic scintillation detectors (Pilot U, BC 418) face each
other on an optical bench at a distance of
about 2.5 meters. A
positron source (sodium-22) in between emits the annihilation
quanta. One is measuring the small time difference between the
arrival of both quanta at the detectors for different positions of the
source on the bench. The set-up is time calibrated using two different
methods to obtain absolute values for the speed of light which can be
determined with a precision of about 0.5%.
Source
PM
Hamamatsu
H2431-50
Scint.
BC-418
(Pilot U)
Scint.
BC-418
(Pilot U)
PM
Hamamatsu
H2431-50
Bench with mm scale
HV
Canberra
3002D
Coincidence
Ortec 418A
POS
POS
Diff. CFD
CMTE 7029A
Diff. CFD
CMTE 7029A
NEG
NEG
Strobe
ns - DELAY
FAST 7800-7
TAC/SCA
Canberra
2145
ADC/MCA
Canberra
Multiport II
Computer
Dell PC
Speed of Light : Required Knowledge
¾ Speed of light history, methods
¾
of measurement
compare organic with anorganic scin-
¾ Today fixed by definition (replaces meter)
¾ Importance of the speed of light
Fast γ-detectors: Pilot U and BaF2;
tillators
¾
Fast photomultipliers
¾
Fast electronics: constant fraction
¾ Goal of this experiment: Training
discriminators and TAC’s
of ultrafast timing methods
¾
Signals and cables
¾ Units and numbers, standards
¾
Fast-slow method (timing and energy)
¾ Physics of positron annihilation,
¾
Compare coincidences with regular
theory and effects
¾ Positron sources
source and with annihilation radiation source (strong angular correlation)
Speed of Light : Tasks and Goals
¾ Set-up of detectors and electronics
¾
using oscilloscope; HV = - 2020 Volts
¾ Set-up of the MCA to 1024 or 2048
displacement (least square)
¾
channels
60Co
2) using calibrated delay lines
source and about 2 x 30 cm distance,
TAC range 50 ns is more linear
¾ Measurement
of
speed
of
coincidence
One spectrum for all peaks !
¾
light:
detector distance = max, source in
center,
Measure two time calibrations:
1) using the Ortec time calibrator
¾ Watch coincidence line using
the
Determine average value of channels /
peak
at
interval
¾
Determine speed of light immediately
¾
Measure the energy spectrum of the
middle channel
¾ Measure peak for variations of
source positions every 20 cm’s, one
spectrum for all peaks ! Stop MCA
and continue without erase !
Determine average channels / time
22Na
source with appropriate settings
¾
WARNINGS
¾
Keep about 1m distance from source
except when changing source position
Time resolution for speed of light (60Co source)
250 ps
Speed of light experiment; 22Na source displacement
20 cm
Speed of light experiment: time calibration using a precision oscillator
10 ns
Speed of light experiment: time calibration using precision delay lines
1 ns