Physics 405, Fall 2002

Last updated 10/31/03

Helpful Hints, corrections, etc.

Here is a link to the NIST Reference on Constants, Units and Uncertainty .
You may find this useful in analyzing your data. This is the last word on how to
properly quote uncertainties in your measurements.

Experiment I: Measurement of the Speed of Light

• hints on laser alignment
• Don't look through the eyepiece unless the rotating mirror is ON.
• There is significant backlash in the vernier - think about how to take all your data while turning the vernier only in one direction.

Experiment II: Experimental Atomic Spectroscopy

• Optimizing the slit widths for the best resolution is best done with the H/D source.
• Turn the flourescent lights off when you are making a scan to cut down on the electrical noise coming from the lights.
• Don't forget to calibrate the display on the microprocessor against the dial indicator on the monochrometer. If you don't, you may run the monochrometer into either its upper or lower limit. If this happens, contact one of the instructors right away so it can be reset.

Experiment III: The Franck-Hertz Experiment

• Give yourself a quick refresher on RC circuits for oscillating signals before connecting the electronics.
• REMOVE THE RC FILTER WHEN YOU CALIBRATE THE 40V POWER SUPPLY!
• Allen suggests turning off the temperature controller just before taking a scan with ACQUIRE (don't forget to turn it back on though.).
• Simultaneously recording the Franck-Hertz tube current in channel 0 and the ramping voltage in channel 1 is tricky. The ramping power supply is floated away from ground by the retarding voltage.  I suggest measuring from ground to the filament, this measures the sum of the retarding voltage and the ramp voltage.
• Think hard about how to fit the peaks, and how to "normalize" the curve to correct for the fact that the overall current rises with the accelerating voltage.

Experiment IV: Determining Planck's Constant

• Take a look at the plot of intensity vs wavelength to find the best peaks to look at.
• Give yourself a quick refresher on RC circuits for oscillating signals before connecting the electronics.
• You might be interested in taking a look at this recent measurement of Planck's constant at NIST in Gaithersburg, MD. See the associated article (21 Sep 98) by E.R. Williams, et al., Phys. Rev. Lett.  Vol 81, (1998) 2404.
• Look at Melissinos (p. 24) for suggestions on how to analyze the data to find the stopping voltage.  Think about this - other methods may work even better!

Experiment V: There Ain't None!

Experiment VI: Measurement of (e/m) for the electron

• You MUST go over the operation of the vacuum system with either Allen Monroe, Tom Baldwin, or one of the instructors before starting the experiment! See also Appendix A in the lab manual.
• Here's a calibration of the Helmholtz coil from 1996, plus a plot showing the residual errors.
• Think about the contribution of the earth's field to your measurement, and how you can account for (or get rid of) it.
• See the updated instructions for operating the vacuum system, replaces Section III of Appendix A.

Experiment VII: Introduction to Nuclear Spectroscopy I

• NOTE: this experiment along with VIII and IX use a data acquisition and software package  specifically designed for nuclear spectroscopy. You should take a little time to familiarize yourself with the program before attempting to take data. See  Appendix C for a brief description of the program.
• For this experiment, and also exps VIII and IX, here is a brief description on using either the Tektronix 2245A or the digital Tektronix TDS360 scopes.

Experiment VIII: Introduction to Nuclear Spectroscopy II

• See the notes in exp VII regarding data acquisition.
• You can find more and up-to-date information about the particular nuclei you are studying at the National Nuclear Data Center at Brookhaven National Lab.  This picture  of the 56Mn->56Fe gamma ray scheme was generated using the MIRD database. Also, take a look at the 228Th spectrum hanging on the wall near the Exp 8 setup for your energy calibration.
• For this experiment, and also exps VII and IX, here is a brief description on using either the Tektronix 2245A or the digital Tektronix TDS360 scopes.
• Section VI (Study of the Decay of ⁵⁶Mn) is confusing.  Your goal here is to (a) calibrate the MCA with the ²²⁸Th source, (b) check the calibration with the ⁶⁰Co source, (c) measure the gamma ray spectrum of ⁵⁶Mn, and (d) attempt to construct a decay scheme from the data in your measured spectrum.  Ignore what the lab manual says about "channel 360" and determine for yourself what gain is necessary to see the peaks in ⁵⁶Mn (up to ~3.5 MeV).  When constructing your decay scheme, pretend that you do not know the answer, and give arguments for why the scheme should be drawn the way you drew it.

Experiment IX: Gamma-Gamma Angular Correlation

• See the notes in exp VII regarding data acquisition.
• Use the fast oscilloscope (Tektronix 2245A or Tektronix TDS360) from exps VII/VIII or exp XI to check the coincidence timing! Follow the signals all the way through the circuit to make sure the signals going into the coincidence circuit are properly timed.
• There is a stronger ⁶⁰Co source inside the little lead can in the lead-lined box. The metal rod (not the block) is the active area of the source. Calculate for yourself how much stronger this source is than the one in the wooden box.
• Before you measure the angular correlation for ⁶⁰Co, it is advisable to first take the singles spectra, sum spectra (output of the amplifier), and coincidence spectra.  Verify that there is a peak in the coincidence spectrum which corresponds to addition of the two gammas observed in the singles spectra.  Sometimes the peak will be cut off due to too high a gain setting on the amplifier or the photomultiplier tubes. 
• The detection efficiency of the two-detector system changes as a function of angle. This affects the overall shape of the data. The effect can be estimated using Monte Carlo techniques. Run the Monte Carlo Program called MC.EXE (written by Prof. CC Chang)  to see the effect. It is located in the DOS directory of programs on the lab computers.
• You will need to fit your data to either Legendre Polynomials, or to equation IX-1 in the lab manual. A sample fitting program, using EXCEL, can be found at p:\p405\Analysis Tools\Excel Tools\ParaFit-mc.xls. This is a parabolic fit with 3 parameters. The data have been entered such that the independent variable is cos^2(theta). It's rather clumsy, but functional.

Experiment X: The Hall Effect in Metals

• You MUST go over the vacuum system with either Allen Monroe, Tom Baldwin, or one of the instructors before starting the experiment! See also Appendix A.
• You will need to take a look at Appendix E on sample preparation, which is not in the lab manual.
• See one of the instructors regarding thickness measurements: there is a relatively new apparatus donated by Dr. Webb.  A manual for it can be found in this Appendix D.

Experiment XI: Cosmic Rays

• To get the solid angle of the detector system, you have to do a four-dimensional integral: you can either use Mathematica, or you can look for the nifty little pascal program written by Prof. C.C. Chang, called INTEGRAL.EXE, which is located on the P405 PCs in  p:\p405\exp11\ .
• Figure XI-2 in the lab manual is OK, but not quite accurate. This one is a little better.
• Use the digital oscilloscope (Tektronix TDS360) to check the coincidence timing.
• BE SURE THE HV SUPPLY is set to NEGATIVE (-)!
• Further information on the various components of cosmic rays can be found in the Particle Data Group's Review of Particle Properties.  Look under Astrophysics and Cosmology, then under Cosmic Rays.