Beta Ray Procedure
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II. Staff Sign-off Sheet (BRA) and Pre-Lab Questions
III. Beta Ray Procedure
VI. Beta Ray Computer Programs
Contents |
Procedure
First read VI. Beta Ray Computer Programs describing the operation of the computer data-acquisition program, so you know how to set the current in the magnet to any value you wish.
Set the signal and discriminator levels
- Familiarize yourself with the relationship between PMT voltage, amplifier gain, and pulse height: Consult the block diagram of the apparatus (media:BRAimage010.gif) and connect the output of the linear amplifier to channel 1 of the oscilloscope. The signal path is PMT to current PREAMP to AMP to a 2 second (-DELAY-LINE BOX to SCOPE (when the scope is triggering internally, the delay-line has no effect, but you will need it later to delay the main pulse). Start with SCOPE parameters of CH 1, 0.2V, 2 (sec/div, CH 1 TRIG, and the Tran-L-Amp Amplifier settings DIFF DL .8, INT 2, GAIN 16. Referring to a sample spectrum, set the coil current (Use the bin setting on the Integrate Single Point program) to a bin corresponding to a high count rate, (the K peak). Observing the signal on the SCOPE, gradually turn up the high voltage (negative polarity) on the photo-multiplier (PMT) until an operating voltage of -800 to -1100 volts maximum is reached. You should see many pulses of various heights. Vary the PMT voltage (always between -800 and -1100V max) and the amplifier gain and observe the effects on the signal gain and noise. Change the coil current to various points on the spectrum and observe how the intensity and height of the pulses change. Note what you see. Remember that you are trying for a maximum signal-to-noise in the pulses. Set the coil current to approximately 0 amps (bin 0) and observe the signal. Theoretically, there should be no signal at zero field. What do you see, and what can you conclude about the nature of the noise in this experiment?
- Familiarize yourself with the operation of the SCA: The normal operation of a PMT will result in small-amplitude noise pulses. We want to use the discriminator on the SCA to reduce or eliminate the noise, permitting the signal pulses which are larger in amplitude to be passed and recorded. Connect the Tran-L-Amp linear amplifier to the SCA and the rest of the circuit elements as shown in Figure 3, apparatus (media:BRAimage010.gif. Switch the SCOPE to EXT trigger. Now the scope is only displaying pulses that pass the SCA, and the 2 second (-DELAY-LINE compensates for the SCA's transit time.
Figure 4a
- With the Single Point Program, find a bin that corresponds to a high count on the sample spectrum. This program is a one part of two of the Beta Ray Scan Program Utilities, Beta Ray Computer Programs. Refer to that section now. Start with the SCA upper-level threshold (UL) set to 10 and the lower-level (LL) set to 0. There should be no signal displayed on the SCOPE. Increase the LL until a signal appears on the scope, this is the minimum LL setting you may use (approx. 0.36), the SCA will not operate properly at a lower setting. Now increase LL well past this setting and observe the output on the scope
- You should see something similar to figure 4.a. In this case, most of the low voltage noise signals are passing through the discriminator and registering as counts on the computer. Now, by slowly increasing the LL (Lower Level) setting, you should be able to obtain an output similar to Figure 4.b. In this picture the lower line and a few of the lower voltage pulses beneath the brightest pulse have been eliminated. This effectively eliminates most of the PMT noise as well as some of the lower momentum pulses..
Figure 4b - The object is to set the LL high enough to eliminate small-amplitude PMT noise while preserving as much of the low-momentum data as you can.
Figure 4c - The AMP gain should be set around 16, but be careful not to clip the top off your signal. If you set the gain to high on the amplifier it will chip or saturate and look like figure 4C. Now using the Beta Ray Scan program, take a series of quick spectra (Bins 0-2500 by 10; 3 sec/bin; save time by clicking on the Stop Early Button, click only once, after it has completed taking data in the 'up' direction at about channel # 2450 (note that the Stop Button takes about 25 second to really stop the program). This should verify your SCA and amplifier settings. With higher LL settings, you should see the lower momentum portion of the spectrum erode. Compare your spectrum to the sample spectrum. If your spectra look too distorted, you may have to repeat step three. Why don't the near-zero-field data disappear with higher discriminator settings?
Observe the effects of statistical fluctuations.
- Using the Integrate Single Bin program acquire 50 or 60 observations each, at one second per integration, for two bins with greatly different count-rates. Save the observations in a file, and use them to calculate the means and standard deviations for the two channels. How do they compare to counting statistics?
- For one bin (one single setting of coil-current), observe the change in fluctuation as you change integration times in a range from 1 to 30 seconds. Based on the lowest count-rate of all channels, for how long should you integrate each channel of your spectrum to achieve a better than 1% error due to statistical factors?
Observe the effect of hysteresis
- Re-read the hysterisis section in the Theory and Background section and the Analyze section of VI. Beta Ray Computer Programs to be sure that you understand how the scan program establishes a hysteresis loop before it begins taking data.
- Take a quick complete spectrum (Bins 0-2000 by 100; 2000-2500 by 5; 2500-4000 by 100; 2 seconds/bin). Observe the shift in the k-peak between the two directions of current. Using the knowledge that the k-peak always occurs at a given magnetic induction (not at a given current), convince yourself that the observed shift in the spectrum between current up and current down is in the correct direction.
Take data using the Beta Ray Scan Program to scan the entire beta ray spectrum
The current in the magnet is scanned up and down which equals one complete scan. The computer controls the past history of the scans so as to follow the same hysteresis curve every time every time. In adjusting scan parameters, it is more important to integrate for longer times, 60 seconds maximum, than it is to include every point in the spectrum. Print out copies of the scans. We are networked in the 111-LAB so you can choose where to print and what printer to use.
