Archived - hpGe Detector (New toy for my lab)

[Jon Rosenstiel]

Found this on eBay...and it worked! It was a little dirty, and the vacuum in the cryostat was weak, but a little elbow grease and 5 hours on the vacuum pump did wonders.

This is an Ortec LO-AX detector. (Low energy, large active area, coaxial detector) The hpGe (high purity germanium) detector element is 51 mm in diameter and 20 mm in length. Covers an energy range from 3 keV to 2 meV. Resolution checks out as good as new. (552 eV FWHM on the 122 keV gamma of Co-57).

The LN Dewar is on the smallish side, 0.4 liters, so it needs to be refilled about every 6 hours.

I’m blown away by this things sensitivity and linearity! Among the peaks a 1.3 hour background check turned up was the 661 keV gamma of Cs-137; from a 10 μc check source 20 FEET away!

Check out the uranium ore gamma specs below to see the difference between my Bicron NaI detector and the Ortec hpGe detector.

Jon Rosenstiel

[DaveC]

Very sweet Jon. A real find. I like the 1/2 keV resolution. It may put a little strain on your calibration process now.


Dave Cooper

[Alex Aitken]

Wow.

[Starfire]

Yet again - 'Drool' - but it landed with the right guy. I think it will be respected, valued and cherished so much more than its brothers in the national institutions and Uni's for all their wealth and budgets. It will open a new deminsion in meterology for you Jon.

I respect your dedication to this amateur quest and marvel at your persuit of quality and precision in your measurements.

This is as real as science gets.

[Alex Aitken]

Next logical step might be an anti compton sheild. Might be tricky to home build but if its possible I'm sure Jon will try.

Is the detector very sensitive Jon or is it just the narrow line width giving high peaks above the noise much more easily?

[Jon Rosenstiel]

Marvin-
I believe you've hit on it.... the narrow line width above the noise makes the detector appear sensitive. (It's all done with "smoke and mirrors")

Geo-
I don't know much about gas detectors, but from what you said I get the feeling that the P-10 gas is expensive?

I know that I will have to plan my experiments carefully so as to make good use of the rather expensive liquid nitrogen. I most certainly cannot afford to have LN on hand 24/7! (It really disappears in a hurry in the + 90F temperatures we’ve been having lately)!

Jon Rosenstiel

[Jon Rosenstiel]

Geo-
I live in southern California... closest LN source is a 10 minute drive. $3.03 / liter plus $6.38 for "energy surcharge liquid". (Whatever that is) It cost me $39.52, tax included, to fill my 10 liter dewar.

Jon Rosenstiel

[Starfire]

Is that the " southern California " with 45*C? )

[Jon Rosenstiel]

Afraid so, John. Where I live, near Disneyland, temps hit 42 C last week. But what's killing me is the humidity! This is the desert... it's supposed to be dry! I think the Irish have something to do with this weather!

Jon Rosenstiel

[Jon Rosenstiel]

Geo- The preamp is built into the detector's housing. Its power cord plugs into the back of the main amp. (Standard nim 9-pin D-sub connector).

I haven't tried it, but I'm pretty sure it would work ok (not great, but ok) plugging the preamp's output directly into the mca's amp in BNC jack.

To get the best possible resolution out of the hpGe detector you're gonna' need a fairly modern spectroscopy amp, its unipolar output plugged into the mca's ADC in jack.

Jon Rosenstiel

[DaveC]

There is a nice summary of HPGe Detectors, by ORTEC, which I found on Google - "hpGe Radiation Detectors". The write up is tilted a little towards their product line, but the pape has a nice reference list at the back with one or two papers in the list discussing basic principles for those not familiar. Among the geometries they discuss, is the LO-AX detector that Jon has.


Dave Cooper

[Richard Hull]

It is 100 Degrees here today in Richmond with near 100% humidity and no relif in sight. They say it will be much worse tomorrow and Wednesday.

LN2 here is $2.42/liter with no surcharge as of last January. So, it is about the same price as beer.

Regardless of whether you buy beer or LN2 for your detector, there is some pleasure associated with the use of both, but in the end your money gets pissed away with nothing left to show for it.

P-10 is basically propane with a little argon in it. It is way over priced. Supposedly, according to the manufacturers, the propane is ultra pure. ( can believe that they pull out any C-14 from the propane)!!

Hooey....I think tank propane and tank argon would be OK for me. I have an eberline alpha detector system with a hand sweeper probe that demands p-10 as well as two flowing gas 2pi proportional counters. (mostly best for low end beta counting of minimal activity samples.) I have yet to fiddle with them much though.

Richard Hull

[JohnCuthbert]

If the propane is petrochemical in origin (and I bet it is) then lying in the ground for a few million years will have removed all the C14.
I'm always amused by folk taking the "home grown" route and thereby avoiding paying big money. Good luck and let us know how you get on.

[Richard Hester]

If I were going to make up some P-10, I'd use lab grade propane without the stinkum. Any commercial grade stuff has the odorant added for safety purposes. I'd hate to have that stuff inside a proportional counter. I would hazard a bet that it drastically affects the conductivity.

[Richard Hull]

Yeah, I have got the same prices and considering it is flowing gas, (meaning it is wasted), I shutter at buying any.

Flowing D2 for a fusor is another matter. In this case you are using it to get neutrons and do fusion. For P-10 you are just counting. Not quite as glorious.

Richard Hull

[DaveC]

Richard - What about recycling the gas? If the pressure is not excessive, a peristaltic-type pump or possibly a bellows might work. (I am somewhat anxious about using flexible tubing pump with a flammable gas, however.) But possibly a stainless steel bellows for the accumulator which could be recompressed to continue the flow, might work.

Dave Cooper

[Richard Hull]

I have yet to see any professional instrument that recyles gas, especially P-10 gas. They all vent the gas. It is up to the user in a lab situation to provide for safe removal. Manufacturers just supply an outlet barb.

There is little reason why the gases could not be recycled, I guess. Purity and return pressure would have to be maintained, of course.

As the hated words say.........." but this is left to the student as an exercise"

Richard Hull

[JohnCuthbert]

With only 5% methane in an inert gas you don't need to worry about flammabillity.

[DaveC]

The 5% to 95% range is considered explosive - if the remaining gases are normal mix atmosphere. For a N2- Methane or N2- propane mix, there would not be any combustion (oxidation), hence generally non-flammable. But a little air leak and that all changes.

Dave Cooper