Fast Neutron Counter

28 10 2010

Dear community,

Please help me evaluate this fast neutron counter:

Is this appropriate for measuring fast neutrons from deuterium-deuterium fusion?

Accurate neutron counts are crucial for both the research and safety.

This meter comes calibrated and is in my budget (barely).

Fast Neutron Counter

12 10 2010

All photos.

I just scored this sweet vintage military fast neutron counter for $370!



Serial A 21

Unit of Radiac Set AN/PDR-47C


By contractor


Denville NJ


Probe, Radiac


Where would I get this thing calibrated?




Neutrons / Second

26 06 2010

Our last fusion run produced results we can use to calculate the neutrons per second emanating from the fusor.

To help with converting from bubbles to neutrons per second I contacted Rob Noulty of Bubble Technology Industries – makers of the bubble dosemeter we are using. He says:

Hi Mark:
Did you run a control detector (to see what natural background bubbles you will get over this time)?  You must subtract these bubbles out assuming you have two detectors of roughly the same response (or you will need to scale).
Please note as well that you have a very small number of bubbles resulting in poor statistics (and a very large error). Based on 4 bubbles, the expected error is roughly 50%.
The calculation is follows:
1.     Divide bubbles by the sens in b/mrem
2.     This will give you the measured dose in mrem
3.     Divide by 3.48 x 10-5 mrem /(n/cm2)
4.     This will give you fluence (in n/cm2)
My detector has calibration: 24 b/mrem (2.2 b/uSv):
Here are my calculations:
This seems to ballpark agree with this fusion calculator:
Question to the community: where do I apply the error factor?
Also, I have yet to factor out the background reading I did previously: 1 bubble in 3486 minutes (2 days  10 hours  6 minutes).

Rob Notes:
There are two conversions, depending on dose conversion table to want use!
USA is going to ICRP-74 slowly, we use NCRP-38.
NCRP-38: 3.48 x 10-5 mrem/
ICRP-74: 4.16 x 10-5 mrem/

Background Reading

21 06 2010

All photos.

Just completed a background reading on the bubble meter. We saw 1 bubble in 3486 minutes (2 days  10 hours  6 minutes).


6 06 2010

All photos.

With the vacuum pump working again, I assembled the fusor and attempted a fusion run last night. It was a day frought with challenges, but in the end the bubble meter saw 4 bubbles in 2 hours and 40 minutes:

This shows beyond a doubt that we have fused the atom. During previous attempts we only produced a single bubble… which suggests fusion, but does not rule out a cosmic ray.

Science Journalist Quinn Norton was at the lab writing a story for Gizmodo. She witnessed and documented the fusion run:

Previously we were having problems with transient voltages spikes or EMFs crashing the data acquisition (DAQ) card. Today was a big test for the new transient voltage suppression system . It FAILED big time. But I learned something in the process.

I began by intentionally creating an unstable plasma to test the transient voltage system.  This crashed the DAQ every time.

Next I disconnected all wires to the DAQ to determine if the interference is coming through the wires or the air:

Without computer control I needed some way to manually adjust  the MFC. I hacked together a quick voltage divider using a 2KΩ potentiometer and a 9V battery:

This proved to work very well.

To control the high voltage from a distance I used the emergency stop button:

This also worked very well. At this point the reactor is completely under manual control. No computer necessary. Which will turn out to be a good thing.

So now we can test the DAQ with no physical connection to the reactor.

Surprisingly, I was able to crash the DAQ every time, even with no wires connected to it!

Quinn noticed the USB hub flickering during the plasma sparks and suggested it may be the failure point. I removed it, and indeed the system seemed less vulnerable to crashing:

At this point the DAQ seems to remain running in the face of sparking plasmas. Good.

Next I tried connecting one channel to the DAQ… a digital output channel to turn the high voltage on and off. I created a duty cycle function in labview to make it easy to bake out the chamber without melting the fusor grid. This is what it looks like running:

Next I bake out the chamber for an hour using a deuterium atmosphere @ 10 mtorr. The high voltage power supply is set to it’s maximum: 30Kv @ 10mA and the duty cycle is set to 75% @ ~ 0.09 hertz.

At first the computer controlled bake out was running smoothly. About 15 minutes in I get a computer crash. Restart. It runs for about 7 minutes and crashes. Try again. 5 minutes and it crashes. The crashes in increase frequency until I am getting nowhere.

At the point I switch the system over to full manual control and begin the metered fusion run. A fresh bubble detector was unboxed and activated.

(so fresh and so clean)

For the main fusion trial the deuterium atmosphere was at 10 mtorr, high voltage set to it’s maximum: 30Kv @ 10mA. The procedure was to run the system at full power until the plamsa became unstable and started sparking. This instability seems to correspond to the grid becoming red hot, so the plasma instability may be due to thermionic emission.

The bubble detector was activated for 2 hours and 40 minutes. The plasma was running for some unknown fraction of that time. 4 bubbles were detected.

Challenges remain for controlling this wily beast with a computer.

Fusion attempt live feed

22 11 2009

We are live tweeting a fusion attempt today. Olivia Koski, a science journalist student is here to document.


2 hours of bakeout prior to metered trial.

Calibration on the bubble detector label: BD-PND, 25 b/mrem(2.3 b/uSv).

Bubble detector is 95mm from the center of the grid.

We got a single bubble during an 8 minute run:

Towards the end of the experiment we noticed a wild outburst of geiger activity while the fusor was _not running_. Not sure what this means. We got it on video:

Using twitter as an experiment log worked very well. It helps capture details you notice along the way with timestamps.

Second Attempt

25 10 2009

Made a second fusion attempt today with a deuterium plasma. Using the new power supply and the mass flow controller together produced very stable plasmas. I tried a variety of voltages, currents, and pressures but no bubbles.

I have two hypotheses:

a) we are producing fusion, but the detector is not showing it.

b) we are not producing fusion because of grid misalignment.

While double checking the bubble detector, I noticed a relevant detail: the bubble detector must operate within 20˚ C to 40˚C. Today the room temperature was 16˚C. The detector has a built in liquid crystal thermometer. Black means the detector is out of range. To correct this, I put the detector in my pocket for 20 minutes. This warmed the detector to 34˚C :


I want to get a geiger counter as a double check for the bubble detector. The geiger counter would respond to x-rays produced during fusion.

But really I think the problem is that our inner and outer grids are completely misaligned. From what I’ve read grid alignment is necessary for “star mode”. And it seems this is necessary for fusion.

It should be pretty easy to fabricate a new/better pair of grids.

Although we didn’t get fusion today, it was a success in other ways. The system is working smoothly. We have stable plasmas with voltages as high as -17kV. The mass flow controller minimized the deuterium use.

The mass flow controller also lets me adjust the gas flow at a safe distance, which is a welcome upgrade.

Here is a video of the deuterium plasma:

New Arrivals

9 09 2009

Got the first bubble detector today:


Unfortunately without the pumps it’s all dressed up and nowhere to go. These have a shelf life of 90 days, and must be stored in their airtight cigar case:


I had to purchase 3 of these. Each will ship as the last expires, so I’ll have 270 days of coverage.

Also got two IGBTs today:


These can switch 200A 600V.


Fuji A50L-0001-0260/A


Pretty heavy duty! I expect to use these on the next run with the superconducting magnet to quickly disconnect the power supply from the magnet.


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