Tuesday, July 15, 2014

Dynode Dawdling Lab Book 2014_07_14

The photomultiplier tube on the NaI detector doesn't seem to be doing quite what it should.  There's a generally held belief, (when I say generally held belief, read, "a professor told me"), that the count rates aren't high enough.  Last week, the case of the tube was arcing to my hand making nice little static shock looking flashes of light.  Consequently, I spent today doing research on the three most likely PMT tubes that are housed within the detector.  Since the tube is surrounded by a mu metal magnetic shield, it can't be inspected to find it's exact type.  I have a call in to the company that purchased the original manufacturer, and hopefully they can figure out which kind of tube we have.  In the mean time, I've turned down the tube bias which seems to have stopped the arcing.  I've also inspected the voltage divider that provides the accelerating potential for the dynodes in the PMT.  This inspection could just as easily have stopped the arcing by moving a component away from the metal housing.  While the divider chain case was off I went ahead and drew a schematic of the circuit.  A partial Cesium137 spectrum was taken this afternoon and it looks promising.

New to the experiment?  Scroll to the bottom to see background and get caught up.

Lab Book 2014_07_14
Lab Book 2014_07_14     Hamilton Carter

I’m working on debugging the dynode divider which appears to have been damaged last week, probably by running too much voltage on the power supply attached to it.  I came across the following spectrum that looks great given the nature of the spectra that may be present in the experiment.  The prediction of the maximum possible radiated energy via h-rays does not mention anything about what the overall spectrum will look like.  If we have a maximum energy of 187 keV, we could also get radiation anywhere below that level.  In the following spectra from the Harshaw manual, you can see that the exact detector that is to be used can see radiation all the way down to the 32 keV Barium K X-ray line.

I found a thesis where the Harshaw 12S12/E was used.  We have a 12S12/3.  I’m not sure how they’re related yet.  In the thesis, a Dumont 6363 photomultiplier tube was used.    There is also a schematic for the dynode voltage divider chain that was used with the Dumont 6363. 

I’ll check these resistor values against the ones in the dynode divider if necessary.  I found a second thesis that mentions using a supply voltge of 900 rather than the 1000 Volts mentioned in the first thesis, (see the figure above).
I also found a paper that mentions the Harshaw 12S12/3 specifically.  The is the model that we have.  The paper mentions another key piece of information.  The aluminum housing of the crystal is 0.5 mm thick and is made of spun aluminum.  The window itself if listed as 0.010 inches thick at another web site.

On a mostly side note, this report is kind of interesting as to what is available for history of physics research.  It’s the equipment inventory for a nuclear rocket project performed by General Aerojet.
In this report, the PMT is listed as a RCA 8054.
In this LANL report written by Ellery Storm, the PMT is also listed as an RCA 8054.

This might be a good reference for supplementing with film detectors.  It was authored by Ellery Storm.

Resistor color code tool used to check resistor values:

Source mentioning that tube is in apparatus
Tube Type
Max Anode/
Max Consecutive Dyn V
2500 V
300 V

2000 V
250 V
1800 V
Not specified

The following pictures detail what the voltage divider chain being used to drive the PMT looks like:

This one details the high voltage input and its associated 55.6 ohm resistor.

This is a side view of the same input.  The identical resistors shown in the foreground are part of the dynode voltage divider chain, see the schematic below.

This picture continues along the divider chain.  Note the capacitors that are connected in parallel across the last few dynode resistors.  They are omitted in the hand drawn schematic below.

Finally, the green wire connects the plate/anode of the PMT to the pulse output that is used by the QVT.
A hand drawn schematic of the divider chain follows:

The pinouts of both RCA tubes and the DuMont tube are all identical and can be seen below.

Dumont Pinout

RCA Pinout

I setup the detector to take yet more data on the Cs 137 source this afternoon.  I’ll have the data in the morning.  Here’s what the spectrum was up to when I left for the evening.
The key difference here is that the tube is biased to 2000 V instead of 2600 V.  The arcing from the tube to my hand has stopped at this bias voltage.

The spectrum looks like it is already exhibiting two peaks after an hour or so of taking data.

Finally, a spreadsheet was made to calculate the voltages on the dynodes.  A sample output is shown below.

These are some of the original source links for the reports mentioned above.  Since Word tries to open pdfs in Explorer instead of Chrome, all the links in the text are to Google Drive locations.

Ellery Storm report
I thought I remembered the name Ellery Storm and it turns out he wrote a book of stories about New Mexico.

Harshaw Manual

RCA 8054 manual original site:

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!

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

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