Thursday, July 10, 2014

Lab Book 2014_07_09 Testing the NaI Gamma Detector

Summary
Callibrtion of the NaI gamma ray detector has begun.  A mistake in the gain and treshold settings made it appear as if the dectector might not be working properly as it gave the same energy spectrum with and without a Cesium 137 source present.  Once the gain and threshold were properly set what appeared to be good spectra were recorde.

New to the lab book?  Scroll to the bottom for background on the experiment.

Using the Photomultiplier Tube power supply
First, turn on the lever switch labeled “Power” on the lower left hand side fo the box.  Make sure to wit fo rhte “Stdby Reset” panel light to illuminate before switching on the “High Voltage” lever switch


To turn on the QVT,  switch on the power switch on the NIM Bin supply on the far right hand side of the bin.


Clearing the QVT
To clear the contents of the QVT, hold the start/stop switch in the stop position while toggling the clear switch.
picture 1 and 2 are the front panel and the cleared spectrum


Spectrum Readings
Taking a reading with the Cs 137 source directly against the side of the NaI crystal.  The following picture shows the setup.

The recorded spectrum follows.


The printer was used to capture the data for keypunch purposes

Here’s the graph of the spectrum from the printed data


After 35 minutes and 23 seconds, I wound up with a very similar spectrum with no source.


Oscilloscope settings used to record spectra.


The spectra with and without the source appear to be very close to identical.

Ideas
·         Try with a different source
·         Try with attenuation to see if the peak moves, (it should)

New information:  the observed peak is the pedestal of the detector.  The gain on the PMT was not turned up high enough to see the signal from the source.  Consequently, whatever noise wsacoming in wasbeing integrated by the QVT and displayed in the lower channels from channel 153 on up.  The threshold was also set too high which is why the channels below 153 are empty. 

To calibrate the photon energy displayed by each channel, I’ll recordth eknown energies of three source, Cs 137,Co 60,and Sr 90.  Using these points, alinear inrepolation of energy per channel will be made
The gate on the QVT determines how long the device should integrate charge signals before allocating them to a counting the appropriate energy channel.   The gate width can be read from a test point on the front panel of the QVT.  The QVT waits for a signal that exceeds the internal trigger voltage and then integrates the signal for an amount of time defined by the gate width.

The gate signal at present is… 0.35 uS.
Threshold was also set on the front panel and was set for -3.41 Volts initially.  The value read on the threshold test point is multiplied by 100.  The new value is -1.49 Volts.

The PMT has been biased with 2600 Volts this afternoon and was at 2000 Volts when the two spectra were taken this morning.

The QVT records one quarter picoCoulomb perchannel.  By looking at the raw pulses on the scope, and considering the 50 ohm termination, you can caluculate how many coulombs you have.

There is currently a factor two attenuation.  It’s in so that we can get all the lines on the same specturum.  The attenuation lets you find out the entire spectrum from the biggest to the smalles t nd then once you know where things are you can find the soure. 

Thers a peak on the right hand edgefrom frobblay the concrete which would be Potassium 40.

The shield will prevet the potassium 40 peak.

Read up o the quarters I the QVT manual to see what they do. Anoterh way of getting bigger sigals is to raise the voltge on the PMT.

The last picture on the camera is the newest spectrum.


Measure the present threshold in the morning.  DONE

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 

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