Speed of Light Speed of Light

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Speed of Light Speed of Light

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

Diff. CFD

CMTE 7029A

NEG

ns - DELAY

FAST 7800-7

Strobe

TAC/SCA

Canberra

2145

POS

Diff. CFD

CMTE 7029A

NEG

ADC/MCA

Canberra

Multiport II

Computer

Dell PC


Speed of Light : Required Knowledge

Speed of light history, methods

of measurement

‣ Today fixed by definition (replaces

meter)

‣ Importance of the speed of light

‣ Goal of this experiment: Training

of ultrafast timing methods

‣ Units and numbers, standards

‣ Physics of positron annihilation,

theory and effects

‣ Positron sources

‣ Fast γ-detectors: Pilot U and BaF 2 ;

compare organic with anorganic scintillators

‣ Fast photomultipliers

‣ Fast electronics: constant fraction

discriminators and TAC’s

‣ Signals and cables

‣ Fast-slow method (timing and energy)

‣ Compare coincidences with regular

source and with annihilation radiation

source (strong angular correlation)


Speed of Light : Tasks and Goals

‣ Set-up of detectors and electronics

using oscilloscope; HV = - 2020 Volts

‣ Set-up of the MCA to 1024 or 2048

channels

‣ Watch coincidence line using 60 Co

source and about 2 x 30 cm distance,

TAC range 50 ns is more linear

‣ Measurement of speed of light:

detector distance = max, source in

the center, coincidence peak at

middle channel

‣ Measure peak for variations of

source positions every 20 cm’s, one

spectrum for all peaks ! Stop MCA

and continue without erase !

‣ Determine average value of channels /

displacement (least square)

‣ Measure two time calibrations:

1) using the Ortec time calibrator

2) using calibrated delay lines

One spectrum for all peaks !

‣ Determine average channels / time

interval

‣ Determine speed of light immediately

‣ Measure the energy spectrum of the

22

Na source with appropriate settings

‣ WARNINGS

‣ Keep about 1m distance from source

except when changing source position


Time resolution for speed of light ( 60 Co source)

250 ps


Speed of light experiment; 22 Na 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

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