RutherfordRutherford ScatteringScattering

LordLord ErnestErnest RutherfordRutherford ofof NelsonNelson NobelNobel priceprice inin chemistrychemistry 19051905 Rutherford Scattering

Rutherford scattering is based on the elastic deflection of charged particles in the Coulomb field of an atomic nucleus. It can be observed and measured by radiating α-particles on a thin metallic foil and measuring the angular distribution of the scattered α’s behind that foil. The intensity of the α-particle beam varies with the deflection angle and the proton number of the used material (see formula). Rutherford’s measurement was the basis of Bohr’s atomic model and the size of the nucleus. In this experiment both the angular and the Z-dependence can be measured in a broad dynamic range. However some corrections due to the thickness of the foil which causes multiple scattering and angular straggling are necessary. Nowadays Rutherford scattering is applied for material analysis and as a cross section calibration in nuclear experiments.

Nucleus

Rutherford scattering formula HV Canberra 3102/2

Si Detector Linear Amp. Preamp Scattering Tennelec Ortec 109A Chamber TC202BLR

Edge/Crossover Log/Lin Timing SCA Ratemeter Canberra 2037A Ortec 9349 Diaphragm Pump Neuberger N813.5 ANE

8K ADC Absorption Computer (PC) Aptec Card Pump and ThinkCentre IBM # 5008 Dewar

Vacuum Pressure Gauge Valve Rutherford Scattering : Required Knowledge

¾ Bohr’s theory of the atom ¾ Deviations from σRuth at higher ¾ Rutherford’s experiment energies : nuclear interaction ¾ Theory of the scattering, two ¾ Radioactive α-emitters, prepara- body problem tion of α-sources ¾ Definition of the cross section ¾ Applications: 1. standard cross ¾ Derivation of the Rutherford section for comparison and

scattering cross section σRuth calibration; 2. RBS-method ¾ Z-dependence and angular ¾ Finite geometry corrections dependence ¾ Physics of particle detectors, ¾ Energy loss and straggling : especially Si-detectors corrections for the measure- ¾ Electronics for spectroscopy ments ¾ Principles of vacuum tech- niques Rutherford Scattering : Tasks and Goals

-5 ¾ Produce vacuum in the range 10 ¾ Measure the line resolution with and mbar (our instrument shows only without foil (angle 0o) 10-3 mbar) ¾ Measure the angular distribution using ¾ Switch on NIM-power the gold foil; the larger angles need ¾ Watch detector signal on longer measuring time, evaluate the oscilloscope and turn on slowly line always at the MCA, consider back- bias voltage of Si-detector (+ 25 ground and uncertainties Volts), angular position of 241Am- ¾ Determine the Z-dependence using the source at 5o , gold foil three foils Havar, Silver, Gold and an ¾ Get a signal from the α-line at angle of 10o about 7 Volts and get the line at ¾ Consider energy loss, straggling and the MCA (set to 256 or 512 double scattering at small angles and channels) correct for those effects WARNINGS

¾ Close chamber window with black cover because Si-detector is light sensitive ¾ Don’t forget to fill the dewar. For overnight runs: fill at least at 10-hourly intervals ¾ After finishing the measurement : hide source behind bar in order to protect the detector, α-source stays in the chamber ¾ Shut down detector bias ¾ Close the gate valve ¾ Never vent the chamber or get help ¾ From time to time regeneration of the Zeolith is necessary, heating up with pumping using the forepump, main gate valve closed. Forepump (diaphragm) often starts only after venting