Physics 576, Week6, Rutherford Scattering Practice
Generally, the procedure for today will be to deliver a 1-MeV proton beam onto a thin
target foil in the 24” chamber and observe Rutherford scattering from the foil at a variety
of angles.
WARNINGS: Si detectors are very fragile. Never touch the surface of the detector; a
fingerprint can cause charged particle energy loss and also affect the electrical properties
of the detector. Dropping a detector or banging it around can also be fatal to the thin
crystal. The detectors are light sensitive and can only be biased in the dark. If biased in
the presence of light they may incur a dangerously high leakage current. The detectors
should only be biased at atmospheric pressure or under vacuum in the micro-Torr region
or below. In the milli-torr region, a biased detector is susceptible to electrical breakdown.
Therefore one should never pump or vent the vacuum chamber with the detector biased.
See point 3) in last week’s procedure for more on how to bias the detectors carefully.
Initial Setup (has already been done for you, but please read to make sure you
understand what is involved).
1) Check to make sure that everything in the 24” chamber is in place. Practice moving
the arms and the target holder with the remote controls in the counting room. Check for
mechanical hysteresis in these systems – if you find some then you will need to take
appropriate measures. Check how much slack the micro-dot cables have and make sure
they won’t get caught on anything when you move the arms remotely. Calibrate the
remote-control set-point to the physical angle of the detector.
2) You will use collimators that are in a box about a meter upstream of the target to
define the diameter of the beam. Open the box, familiarize yourself with the collimators,
and record any relevant information. There should be two collimators: a “defining
collimator” that defines the diameter of the beam and a “cleanup collimator” that blocks
any beam that might be scattered from the edges of the defining collimator. You will also
be using these collimators for beam tuning by reading the electrical current from them
during tuning. Check that they are connected electrically to vacuum feed-throughs and
make note of where they are output.
3) You will use the Faraday Cup downstream of the target to stop the beam and measure
the corresponding electrical current, which can be used to determine the number of beam
ions hitting the target. Later, you will bias the Faraday cup with a dry cell (DC battery) to
ensure that electrons emitted when the beam hits the cup do not escape (this would affect
your current reading). Do not bias the Faraday cup yet – you do not want it to be biased
when you pass through the milli-torr region.
4) Check that the signals from the collimators and the Faraday cup are connected to the
patch board and follow these cables through to the counting room. The Faraday cup
should be connected to a beam-current integrator (BIC) that acts as an ammeter. The
digital output BIC should connected to a scalar unit that counts the integrated current (i.e.
total charge) by assigning it a number that accumulates over time. Using a constant
current source, determine the units of the scalar in Coulombs for various settings of the
maximum scale on the BIC. Another output of the BIC is connected to the control room
where it can be used as a beam-tuning diagnostic. The signals from the collimators can
also be read using ammeters in the counting room. Check this using the current source.
5) When the system is pumped down, apply the bias to the Faraday cup by connecting the
dry-cell battery to the guard ring.
6) Deliver a 2-MeV beam of protons to the target in the 24” chamber. Detailed
instructions can be found in the “Accelerator Operations Manual”. You will have to start
the DEIS source, and tune a mass 1 beam on deck and then to the LE cup. Try elevating
the deck to 100 kV this time to improve transmission. Aim for about 100 nano-amp of
current going into the tandem. Start the tandem, charge the terminal and tune the beam to
the object cup. Make sure that the analyzing magnet is set-up for your ion of interest.
Double-check that the beamline to the scattering chamber (and the chamber itself) are
evacuated and then tune your beam through the collimators to the BIC.
The two detectors are covered with shielding with apertures of 0.7 cm of diameter aiming
towards the target and at a distance of 17 cm from it. The angle with respect to the beam
can be varied from the outside.
Top view of the 24”
scattering chamber
open.
Experiment
You will be given some known targets at given positions in the ladder and some
unknown targets.
1) Set the target wheel so that the thin Au target will be irradiated. Calibrate your
channels into energies using this foil and the two-body kinematics program at:
http://faculty.washington.edu/agarcia3/phys576/links.shtml
2) Use your calibrations and Rutherford scattering on the mystery foil to identify what
element it likely is.
3) Acquire spectra at 110, 130, and 150 degrees with respect to the beam. Make sure to
save each spectrum.
Things to think about:
Do you think you were scattering dominated by Rutherford (or Coulomb) scattering for
the different foils? Why or why not? Do you have enough information to determine
absolute values for the differential cross sections? (It is ok to assume that the beam
current on target is the same as that read in the Faraday cup called “the Flap”. Use the Au
target to compare the number of counts you observe with what you would expect based
on the Rutherford scattering cross section.
Do you expect the energy of the observed peaks will change linearly with the incident
beam energy (say, if you increase the beam energy by 10%, do you expect the peak
energy to increase by 10%?)
4) Make rough comparisons of the ratio of the number of scattered particles at 150
degrees divided by the total incident charge for foils of Au and C. Indicate which one is
larger and by what factor. Does this roughly agree with what you expected?