Compton EffectCompton Effect

Arthur Holly ComptonNobel price 1927

Arthur Holly ComptonNobel price 1927

The Compton Effect

The Compton effect is one of the most fundamental effects: scattering of high energetic photons off a free or bound electron. It is important for the understanding of basic quantum mechanical effects.In this experiment the energy shift of the scattered gamma radiation and the scattering cross-section are examined in relation to the scattering angle. In the present set-up the probes are ring-shaped and one gets a much higher scattered yield than using the standard geometry.Using angles from 20 to 165 degrees the Compton relationship can be determined for the energy shift with an error of 3%. The validity of the Klein-Nishina formula for the effective cross-section can be proven with an error of approximately 10%.

PreAmpCanberra 2007B

HVTennelec

952A

AmpTennelec

TC241

Canberra Multiport II

Computer Dell PC

Bench with mm Scale

NaI Detector (inside lead shielding)

Shadow BarMetal Ring

Source and Holder

Voltage DividerCanberra 2007

Compton Effect

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Compton Effect : Required Knowledge

Theory of the Compton effect

= Scattering of γ’s off electronsEnergy and momentum con-servation

Klein-Nishina formula

Interaction of γ’s with matter

Absorption of γ’s in matterCorrections required for the chosen set-up : Multiple scat-tering; absorption in the ring samples; variable solid angles

Principles of Gamma detectors,

especially NaI(Tl) detectors

Principles of γ-spectroscopyPhotomultipliers and basic

electronics

Advantage of collimated detec-

tor geometry (compare with

anti-Compton shield)

Advantage of ring samples

(compare with other methods

for the Compton effect measure-

ment)

Compton Effect : Tasks and GoalsCompton Effect : Tasks and Goals

Set-up of detector and electronics using a weak 137Cs source, put source above the end of the shadow bar. Never change the detectorCalibrate the detector with 662 keVline and Ba Kα-lineDetermine detector resolutionSet MCA to 512 channels Align source, sample, shadow bar and detector very precisely to the optical axisFirst think about an optimal setting for the scattering angles, work out a scheme

Measurement of 2 angular distributions using the strong 137Cs source for Cu or Al:

1. Energy of scattered γ’s in depen-dence of scattering angle

2. Angular distribution of the scattering

cross section (Klein-Nishina)

Always use differential measurement

with equal time for ring-sample-in and

–out spectra; use subtraction (stripping)

function

Determine uncertainties

Compare Compton effect for Cu and Al at

one convenient angle

WARNINGSBe careful with bad.Shut down spectroscopyNeverRemove source after measurement