Physics 405, Spring 2007
Last updated
Jan 23, 2007
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 (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 best to fit the peaks or valleys, and how to
"normalize" the
curve to correct for the fact that the overall current rises with the
accelerating voltage. You can either use the Mathematica template
or fit the peaks by hand, but be sure honestly estimate your
uncertainties either way.
Experiment IV: Determining
Planck's Constant using LEDs
Experiment V:
- This experiment is still under construction
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 the handout at the lab station and the link in
the course web page (look under Exp 6).
- Here's a
calibration from 1996, plus a plot showing the residual
errors. Refitting this data (2003) shows the
uncertainties: intercept of (0.250 +/-0.009) G and the slope of
(47.3 +/- 0.1) G/Amp. The whole calibration was recently redone
and a plot is posted by the experiment. The results of this
calibration are intercept =
(0.3 +- 0.1) G and slope = (43.7 +- 0.1) G/Amp.
- Think about the contribution of the earth's field to your
measurement, and how you can account for (or get rid of) it.
- There is also a calibration of the high voltage supply -- the
analog meter is off by about 7%. See the picture on the wall in
the lab and the notes under "Documentation".
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. The
software is called MAESTRO.
- For this experiment, and also exps VIII, IX and XI, here is a
brief
description on using either the Tektronix
2245A or the digital Tektronix
TDS360 scopes.
Experiment VIII:
Introduction to Nuclear
Spectroscopy II
- THIS EXPERIMENT IS NOT AVAILABLE FOR SPRING 2007.
- 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, IX and XI, here is a
brief
description on using either the Tektronix
2245A or the digital Tektronix
TDS360 scopes.
Experiment IX: Gamma-Gamma
Angular Correlation
- See the notes in exp VII regarding data acquisition.
- The lab manual suggests using 1300V for the detector. Use 1200 V
instead., and set the discriminator thresholds to their minimum (30
mV). This will ensure that, for the 60Co data, you won't saturate
the linear fan/in that combines the signals from the two
detectors. When you are taking data with 60Co, look at your
spectrum with "pulse inhibit" mode -- you should see two peaks. If you
see only one, then you are likely saturating the linear fan-in, which
has a maximum output voltage of -1.6 V.
- 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.
- The detection efficiency of the two-detector system changes as a
function of angle and depends on the distance from the source to the
detectors. This affects the overall shape of the data. Prof. Chang has
written a Mathematica notebook to compute the effective detector
efficiency. The
effect can also be estimated using Monte Carlo techniques.
- 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. If
you need to remember the definition of Legendre Polynomials, you can
find them in wikepedia at this link.
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! Take
a look at Appendix
E on the vacuum system and sample preparation.
- See one of the instructors regarding thickness measurements:
there is a relatively new (and very expensive!) apparatus donated by
Dr. Richard Webb. A manual
for it can be found in this Appendix
D, but these measurements must be done by Tom Baldwin, so you need
to schedule an appointment with him in advance to do it.
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 Mathematica notebook written by Prof. C.C.
Chang. You can find it on the course website under Analysis Tools
/ Mathematica Tools,
and on the P drive on the lab computers.
- Figure XI-2 in the lab manual is OK, but not quite accurate. The
level adaptor after the coincidence box is no longer necessary.
- 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.
Experiment
XII: Nuclear Magnetic Resonance
- This experiment is still under construction, but may be close to
working for Spring 2007. By special permission only until further
notice.