609 thermal neutron pencil dosimeters

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Jim Kovalchick
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609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

In my Christmas loot I came away with three old model 609 pencil dosimeters. They are normal ion chamber direct reading dosimeters that are boron lined. According to the 1954 AEC Supplement, the range is "0 to 2X permissible daily dose units based on an allowable daily dose of 1750 n/cm2/sec. for an eight hour day." The gamma dose to move the needle is apparently extraordinary.

I'm a little too busy lately to try getting the needle to move with my fusor. Does anyone have any experience with these? When I get a chance, I will report results of some tests. In the mean time, I think they are cool old items for the collection.

Jim K
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Richard Hull
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Re: 609 thermal neutron pencil dosimeters

Post by Richard Hull »

Really a cool item! I have never see or heard of those direct reading thermal neut dosimeters. I have a gang of regular pen dosimeters that read gamma doses. You have a real rarity there.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

Yesterday I performed a simple gamma sensitivity test of the 609 dosimeters. I put two of the neutron dosimeters and two gamma/x-ray dosimeters (0-200 mRem) in close proximity to a Cs 137 source. After about 18 hours, the regular dosimeter closest to the source showed a deflection of 50 mRem, and the other 30 mRem. Neither of the neutron dosimeters showed any accumulated dose from the gamma field.

These results are consistent with the reports contemporary to the 50+ year old documentation stating that the 609's are pretty insensitive to gammas.

My next tests will need to involve neutrons of course.

Jim K
Alex Aitken
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Re: 609 thermal neutron pencil dosimeters

Post by Alex Aitken »

I ran some numbers and they suggest a 609 with 5.5cm of HDPE moderator around it would get a flux of between 7.6e-5 and 4.2e-4 thermal neutrons per square centimeter per source neutron, assuming a 10cm diameter fusor and depending on how strongly the detector absorbs.

60mrem being 1750 neutrons per second for 8 hours, half the scale, would be ~33 hours for a fusor making a million neutrons a second.

So a few hours for a 10^6 or a few tens of minutes for a 10^7 fusor might produce a visible deflection.
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Jim Kovalchick
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

Alex,
I had done some similar calculations more simply based on a belief that my fusor can produce a neutron field between 1 and 2 mRem/hr. I assume that i will need to do several runs to accumulate a deflection I can see because I only run 20 to 30 minutes at a time.

Thanks for your math.

Jim K
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Richard Hull
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Re: 609 thermal neutron pencil dosimeters

Post by Richard Hull »

Don't forget the PNC-1 you own! Use the PNC-1 charge I just snagged.

viewtopic.php?t=14710

If you have a stable 1600 events per minute on your PNC-1 from your fusor, that equates to a flux of roughly 131 n/sq cm/sec at the location of the moderator/detector.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

Richard,
You would think I would remember that PNC chart since the one you have is a copy of the one that was taped to the side of my PNC when I bought it. Lol.

My fusor pounds out 2000 cpm on a PNC without effort and almost 4000 when I'm pushing.
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Jim Kovalchick
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

I ran my fusor for the first time in over a year. I spent a little time conditioning the chamber, and once I was able to sustain at least a thousand neutrons a second on my PNC-1, I started irradiating two of the 609 dosimeters. One I kept near the chamber but bare. The other I placed in a plastic jar of HDPE granules. I ran the fusor twice for a total of about 50 minutes. The deflection on the dosimeter in the moderator was about 3 to 4 mRem. The unmoderated dosimeter showed no deflection.

Pretty good for a neutron dosimeter that is likely close to 50+ years old.

Jim K
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Jim Kovalchick
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

Today's test of a 609 dosimeter went as follows:

Moderated dosimeter starting: 12.5 mRem
Moderated dosimeter ending: 20 mRem

Bare dosimeter starting: 10 mRem
Bare dosimeter ending: 10 mRem

Total fusor run time: 25.5 minutes
Integrated neutron events on PNC-1: 41,108 counts

Steady state conditions:
Operating voltage: -51 kV
Pressure: 0.0148 mbar
Current: 14.8 mA
Neutron event count rate: 2000 cpm

Highest neutron count rate seen 2400 cpm

Both the PNC 1 probe and the 609 dosimeter were approximately 6 inches from the center of the chamber

The average count rate for the duration of the dosimeter exposure was approximately. 1600 cpm. On the 1964 PNC conversion chart, for 2.45 MeV neutrons this count rate equates to a dose rate of 17 to 18 mRem/hr. For a 25.5 minute run then, the expect dose at the point of the probe would be 7.2 to 7.7 mRem. This compares pretty favorably to the 7.5 mRem on the 609.
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Richard Hull
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Re: 609 thermal neutron pencil dosimeters

Post by Richard Hull »

A great full data report. The one dosimeter with the ~7 mrem total dose is the one to use.

All pen dosimeters are subject to error over time.

We have a member in our local HEAS group who often makes the trip from Hampton, VA. for the monthly meetings. (~70 miles)
Many of you who have attended the last 3 HEAS big October conferences have met him. (Michael Griffin). He has worked for many years for Newport News ship building. They build and repair all the Atlantic fleet nuclear vessels.
He has worked as their radiation instrument lab repair and calibration specialist.

He has told me stories of his having to throw out hundreds of pen dosimeters when he does a massive cal. check on their thousands of such dosimeters.
For those not familiar with dosimeter construction, I will elaborate.

The pen dosimeter, as a calibrated electrostatic instrument, is a sealed system. The principle of its operation is over 3 centuries old, but the precision displayed in this dosimeter is all 20th century refinement. The pen dosimeter is not evacuated, but generally filled with dry nitrogen or dry air to atmosphere. They are charged to make a fine quartz fiber move to the zero point. Thus, they are an air dielectric capacitor.

According to Michael, there are two common failure modes. All of the dosimeters fail due to mechanical shock. This shock often creates a leak in the sealed nature of the capacitor's internal gas pressure by barometric pressure changes and possible moisture entry. It doesn't take much to upset the accuracy. Finally, the quartz fiber can become displaced. It only takes a fraction of a millimeter change in the fiber to throw the reading off. For Michael, all dosimeters go to the trash for any reading in error by 10%.

For most of us, such dosimeter calibration is not an issue. We just want better than a +/-50% idea of what the true dose was. According to Michael, if the dosimeter reads at all, he has noted a worst case scenario of a 25%-30% error, almost always as a higher reading... (leaking - capacitor discharges faster that it should moving the needle up range).

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

Richard,
All three of my 609 dosimeters are good, not just the one that showed change during the last test. The dosimeters only respond to thermal neutrons. The dosimeter that did not respond during the test behaved as expected because it saw only fast neutrons.

Jim K
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Richard Hull
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Re: 609 thermal neutron pencil dosimeters

Post by Richard Hull »

I am confused you show two moderated dosimeter readings with different readings. Were these from two different dosimeters?

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Jim Kovalchick
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

Richard,
Now I see how I confused you. I have edited my post to show that there was only one moderated dosimeter and the readings are before and after. Thanks for pointing out my error.

Jim K
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Re: 609 thermal neutron pencil dosimeters

Post by Jim Kovalchick »

One thing I want to clarify for those who may be following this thread, is that comparing dose derived from a PNC-1 with dose measured by my model 609 dosimeter buried in HDPE granules is very much like apples and oranges. I used curves for the PNC-1 that give a dose rate for a given count rate corrected incoming neutron energy. That cannot be assumed to be the same dose that the pencil disimeter sees. The HDPE granules will likely behave differently than the paraffin in the PNC-1. Further, my granules jar is not lined by cadmium to prevent reflection of thermal neutrons into the fluence.
I expect in the coming days to try to correlate what I see on the 609 dosimeter with a thermal flux. I will use the data I already collected first and after I get back in the lab, I intend to see what can be done to correlate the numbers to fast flux. My ultimate goal is a correlation for this dosimeter type to neutron production rate of D-D fusors much like bubble detectors but easier. I'm not sure how many of the 609 detectors are out there to be had by fusioneers though. I see that Andrew Seltzman has an untested one for sale in his eBay store for a good bit more than I paid for mine, but considering the price of perishable bubble detectors, it could be a low cost option for someone once a correlation is made.
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