Wednesday, August 13, 2014

We have 32 keV! NaI Detector Characterization, Lab Book 2014_08_13

Summary:

Spent the day characterizing the NaI detector.  The cool bit is that the 32 keV peak from Cs137 is visible in the spectrum.  It was distinguished from the background and the pedestal by subtracting a background spectrum from a spectrum with the source.  With the background subtracted out, the peak was clearly visible.  This is important because while the maximum predicted x-ray energy from the experiment is in the range of 150 keV, the radiation may be over a spectrum that contains lower energies.  The smaller the energy we can detect, the better.

It looks like the detector signal may not be linear with the PMT running at the high gain necessary to resolve the 32 keV peak.  This isn’t entirely unexpected.  Tomorrow lower gain tests will be run to see if the detector is any more linear at lower gains.  Linearity in this case means can the calculated voltage per channel on the detector predict where other peaks in the spectrum are.  For  more on this, read the entire entry, and stay tuned for a separate post on the basics of the detector and the PMT .

If you’re new to the experiment, then scroll to the bottom for all the background..


Background run 9:10 AM.

Bias
1400 V
Gate Wind0ow
0.5 uS
Threshold
1.5mV
Attenuation
6 dB
Data set
HBC_0002
Source
Background
Start Time
9:10 AM
Stop Time
9:41
Date
2014_08_13
x-y scope V/div
1, 0.5
Shielded?
No
Tube/Detector
Harshaw B-

Notes:  The QVT display/printer has to be bumped to print the first few channels.  I’m not sure if there’s a data inaccuracy to accompany this behavior or not.  When the start cursor is positioned at 0 and the print start button is pressed, the printer only prints the first four channels.  By advancing the cursor one position at a time you can get the printer to print additional channels.  After about three iterations of this process, the printer prints the rest of the data in the capture range correctly.  In the first ten channels, there is one more channel printed than fits in the range and there appear to be duplicates of the counts for two channels.
Spectrum picture:





Shielded Source Run
Bias
1400 V
Gate Wind0ow
0.5 uS
Threshold
1.5mV
Attenuation
6 dB
Data set
HBC_0003
Source
Cs 137
Start Time
9:59 AM
Stop Time
10:32 AM
Date
2014_08_13
x-y scope V/div
1, 0.5
Shielded?
Yes
Tube
Harshaw B-

Notes:  Run HBC_0003 was done with the source taped directly to the center of the detector window.  The plan is to determine the count rate from the detector using this data.
Spectrum picture





Shielded Background Run
Bias
1400 V
Gate Wind0ow
0.5 uS
Threshold
1.5mV
Attenuation
6 dB
Data set
HBC_0004
Source
Background
Start Time
10:42 AM
Stop Time

Date
2014_08_13
x-y scope V/div
1, 0.5
Shielded?
Yes
Tube
Harshaw B-

Background Picture:





Source Signal minus Background from the previous two runs.



Made a  few rough estimates of the voltage per channel based on the assumption that the first peak shown above is the 32 keV Barium K peak.   The first calculation was done assuming that every channel down to 0 had useful information.  It gave the following results
channel
Count
Voltage
80
5633
32000
400
980
9656
662000
675.5102
%error
29722.449
32 value on 662 V/channel
0.071173
377600
662 value on 32 V/channel
0.429607


The second method assumed that all the channels were useless below the pedestal peak of channel 36 and through them out.  The results had less error:


Assume a pedastal at 36 and recalculate
pedestal
36
channel
Count
Voltage
80
5633
32000
727.2727
980
9656
662000
701.2712
%error
30855.9322
32 value on 662 V/channel
0.035752
686545.455
662 value on 32 V/channel
0.037078


Finally, the last method is probably the most correct one and assumes the channels are linear and the peaks are at the correct locations.  It just constructs V/channel as the slope of a line between the two peaks.

rise
630000
run
900
V/channel
700
offset
-24000
pedestal voltage
25200
Barium K
32000
662 line
662000

Troublsome question of the day
Why does the 32 keV peak have fewer counts than the 622 keV peak with the higher gain setting?  Can this be reproduced?  Keep an eye on this during tomorrow’s low gain linearity test.

The values for the two edges of the Compton plateau were calculated using the last slope and offset determined above.  The equation was

$channel = \dfrac{energy-offset}{V/channel}$

The results indicate that the gain may not be linear at the current settings.
Compton peak
291.4285714


Compton Plateau
720


The measured channel values were about 311 and 658 respectively.

Background
Hirsch's theory of hole superconductivity proposes a new BCS-compatible model of Cooper pair formation when superconducting materials phase transition from their normal to their superconducting state[1].  One of the experimentally verifiable predictions of his theory is that when a superconductor rapidly transitions, (quenches), back to its normal state, it will emit x-rays, (colloquially referred to here as H-rays because it's Hirsch's theory).

A superconductor can be rapidly transitioned back to its normal state by placing it in a strong magnetic field.  My experiment will look for H-rays emitted by both a Pb and a YBCO superconductor when it is quenched by a strong magnetic field.
This series of articles chronicles both the experimental lab work and the theory work that’s going into completing the experiment.

The lab book entries in this series detail the preparation and execution of this experiment… mostly.  I also have a few theory projects involving special relativity and quantum field theory.  Occasionally, they appear in these pages.

Call for Input
If you have any ideas, questions, or comments, they're very welcome!

References

1.  Hirsch, J. E., “Pair production and ionizing radiation from superconductors”, http://arxiv.org/abs/cond-mat/0508529 

No comments: