Archive for the ‘Projects’ Category

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A Simple Spark Detector for Alpha Particles

September 17, 2011

Back in May, Explora!, the local science center for which I occasionally volunteer, referred me to the local public TV station to lead a weekend “Science Cafe”.  The discussion subject was lightning and its connection with cosmic rays.  Trying to augment my usual hands-on electrostatics program with something perhaps more topical, my mind wandered back to a fascinating radiation detector that I’d first encountered in an embodiment built by the consummate craftsman Tim Raney of Richmond, Virginia: an open-air spark counter for alpha particles.

In this type of detector, thin negatively-charged wires are strung through atmospheric air above a planar anode, and sparking occurs when strongly ionizing radiation like alpha particles passes through the high-field region near the wires.  The concept was first described by Chang and Rosenblum in Phys. Rev. 67 (1945).  Click to download the paper.  My version is pictured above, the left hand photo showing its response to a radium source from a Walkie Record-All and the right hand photo the response to a Nuclespot 5-mCi Po-210 source.  Note that this is not a traditional spark chamber; it’s much simpler than a spark chamber because it is self-triggering. It also only responds to alpha particles—no beta or gamma sensitivity at all.  (I should also mention that it is not closely representative of the runaway relativistic breakdown mechanism postulated to trigger lightning, although it does obviously exploit the ionization effects of radiation to trigger avalanche breakdown.)

Construction and operation details are discussed in the video below:

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HPGe Detector, Part I: Repair

September 16, 2011

A high-purity germanium (HPGe) detector is the ultimate instrument for energy spectrometry of gamma radiation.  For the nuclear hobbyist, an HPGe opens a window into a fascinating realm of  home-accessible, low-intensity nuclear reactions that are obscured by background in other detectors lacking the superlative resolution Weak alpha sources available without a specific NRC license can be used to detectably excite (a,n) and (a,p) reactions attended by emission of gamma rays from product nuclei.  Radioactivities induced at the fractional Bq level by weak (a,n) or DD fusion neutron sources can be identified.  The downsides of HPGe detector ownership are obvious to most amateur scientists who have considered them: they’re fragile, consume liquid nitrogen, and—perhaps most significantly—require multidisciplinary knowledge to return to operation.

I was kicked into these uncharted waters when Taylor Wilson sent me an older 2″ Ortec coaxial HPGe detector in unknown condition, and I hesitantly began an effort toward making it work.  Right away I knew Lady Luck hadn’t smiled on me: the input FET was blown.  As I detail in the gallery below, I replaced it with a $2 Japanese audio FET, rigged a vacuum pumping scheme for the Dewar, adjusted the preamplifier, and—voila!—the thing works now, ultimately providing about 1.7 keV FWHM at 662 keV.  From my limited experience repairing an HPGe detector I can’t generalize too much, but perhaps other amateur nukeheads will find encouragement in the success story documented here.

Gallery 1: Teardown and Repairs (click any image for larger captioned version)

 

The following steps comprised my path to a working detector.  Additional details for some procedures can be found at TRIUMF’s website.  To make these repairs, you need an oscilloscope, an MCA, an electrometer, some NIM-standard electronics, and a high vacuum system.

  1. Demount and test the HV filter.  Jon Rosenstiel has found the filters to be a weak point in his repair experience.  Not only will blown resistors and capacitors in the filter prevent the detector from operating, they can cause failure of the input FET.  Make sure the filter’s through resistance is a stable high value (200 MΩ in my model).  These filters do not appear easy to replicate or repair, so if yours is bad, you can pretty much count on spending $500 for a new one.  Nice to know up front before getting too involved in the project!
  2. Test the detector’s preamplifier.  With low-voltage power applied from a NIM bin (but no HV bias), monitor the preamp output for noise on an oscilloscope or MCA.  At room temperature, there will be lots of thermal noise if the FET is alive.  If you’re lucky and your FET checks out, skip the next two steps.
  3. Replace the FET.  You can either pay hundreds of dollars for a new one specially culled by the manufacturer…or you can take a little pot luck on a $2 off-the-shelf part.  For relevant noise and capacitance information on specific commercial FETs, Amptek’s note here is a must-read.  (I initially tried a pair of 2SK152s in parallel, having made questionable assumptions about the crystal capacitance.  Later, when I tried a single 2SK152 transistor, I did not obtain a measurable difference in system noise.)  Take apart the detector head and break the main vacuum o-ring seal on the detector cap.  Solder in the transistor(s) using no flux.  Use a clip lead to ground the crystal HV electrode during this procedure to protect the FET.
  4. Check for high voltage clearance between the cap and the crystal package.  Sometimes there is a thin (0.01″) plastic spacer sheet interposed between–check it for burns or holes.  Damaged plastic sheets may be replaced with the plastic from a clear binder cover (from an office supply store), carefully washed and dried.
  5. Evacuate the Dewar.  Even if the FET is OK, Dewars tend to go soft over time…and that puts the FET in jeopardy because of low-pressure HV breakdown.  Preemptive attention to the vacuum may even be warranted.  You can buy an evacuation attachment from the manufacturer for hundreds of dollars, or you can drill a hole in the Dewar wall (carefully! slowly!) with a standard jobber drill and epoxy a vacuum fitting through it like I did.  Whatever you do, make damn sure the vacuum is good (< 10 mtorr) and will stay good.  Whether to continuously pump or seal off is your choice, but I do the former.
  6. Remount the HV filter and preamp components.  Supply power to the preamp (but not the HV bias!).  If you have an Ortec detector, adjust the preamp charge loop per these instructions.  Failure encountered in this procedure probably indicates a blown FET, but I am told the hybrid ICs on the Ortec preamps go bad sometimes too.  Leave the cover off the preamp; the charge loop procedure (and PZ procedure) will have to be revisited once the detector is cold.
  7. Obtain liquid nitrogen.  Pricing in small quantities is ~$1.20 / liter, so don’t get ripped off by opportunistic asshats at the welding shop who smell teh noob.  Some dealers freak if they see you driving a Dewar around in your passenger car.  If you do take a Dewar in your car, make sure it is strapped in so it can’t spill, and roll the windows all the way down for ventilation.  My 30-liter supply Dewar weighs 83 lb full and sits very nicely in the back seat of a sedan.
  8. Wait several hours after filling the detector Dewar for the detector to be operable.  You can observe the decrease in thermal noise from the preamp output as the detector and FET cool down, and you can periodically readjust the charge loop circuit to track 0 millivolts per the above instructions as the temperature drops.  This adjustment will stabilize when the FET gets cold.  In my system, the process takes just under 1.5 hours.  I recommend waiting several hours before applying bias.
  9. Give it a try: Turn on a variable HV bias supply set at 0V initially.  Use an oscilloscope or MCA to monitor the preamp output.  Approach a radioactive source to the detector head.  Counts should appear even with bias at 0V due to the photovoltaic effect.  Raise the bias to ~100V.  Noise should decrease dramatically.  Keep pushing the voltage while collecting spectra from your favorite gamma source.  At this point, hopefully you’re witnessing your new toy’s sick resolution.

Gallery 2: Testing and Initial Operation (click any image for larger captioned version)

 

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Refining Uranium by the PUREX Process

September 18, 2009

PUREX_0PUREX is the major chemical technique for recovering uranium from spent nuclear fuel. Based on the highly-selective extraction of uranyl nitrate from aqueous solution by tributyl phosphate  (TBP) in a nonpolar organic solvent, the technique is straightforward for home chemists to exploit in order to refine their personal uranium stockpiles.  The photo illustrates the supplies used in the following procedure: nitric acid, tri-n-butyl phosphate (from QualityBiological.com), Kleen-Strip 1-K kerosene (Home Depot), and 4.8 g of homemade uranyl oxide.

Caution: the PUREX procedure involves intimately contacting nitric acid with highly-flammable organic material!  Work with small quantities.  Concentrated acid will form explosive oils, so always dilute it to 6M or less.  This discussion presupposes essential safety understanding of the chemicals and techniques involved.

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New Crusher for Uranium Processing

September 11, 2009

crusherTime to kick it up a notch in the uranium kitchen, since I got tired of crushing ore solely by hand with a hammer.  The new equipment to turn big rocks into very small rocks consists of a three-inch jaw crusher made by Al Yates, coupled to a 3.75-horsepower Briggs & Stratton Model #094202 gas engine.  To match the engine’s 3600 RPM at full throttle down to a safe speed at the crusher cam (and store some rotational energy for particularly resistant rocks), a 1.75″ pulley is used on the engine shaft and a heavy 11.75″ cast-iron pulley on the crusher shaft (both from McMaster-Carr).  The belt is a standard A50 size.  The whole deal is mounted on a wooden base.

This crusher can produce about 15 kg (a two-gallon pail) per hour of fine rock flour from 1-3″ Utah uranium ore.  It would take many tedious days to accomplish this with a hammer, seives, and a ball mill.  Now the slow step in my artisanal mining and processing scheme is acid leaching, and the attendant gravity filtration of sediment in the leachate.

Coming up soon…a look at the PUREX process—solvent extraction of uranyl nitrate into an organic phase.

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RF Ion Source

February 8, 2009

I have been working on an ion source to support my next fusor and other small accelerator projects.  Criteria for this source were that it had to be easy and inexpensive to fabricate myself with common components from reliable sources.  My goals were to obtain high beam current and long service lifetime.  I settled on an RF ion source  concept with specific influences from Kiss and Koltay (1977).  Tests of the prototype indicate stable sub-milliampere currents of deuterium ions over hours of operation.  Cost (excluding RF and vacuum equipment): about $250.

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ion_source_modRF ion sources function by extracting ions from radiofrequency electrodeless discharges.  These sources can deliver high-purity beams of atomic H+ ions.  The vessel supporting such a discharge can be as simple as a glass test tube surrounded by an inductive coupling to the RF supply, as my design at left illustrates.  The discharge is “enhanced” with the field from a strong magnet.  Some builders attempt to exploit specific enhancement effects, e.g. electron cyclotron resonance or helicon phenomena.  My goal with the magnet is just to promote generic electron trapping / heating, possibly by the above-mentioned modes if conditions are appropriate.  Components, with suppliers’ names and stock numbers, are provided in the drawing.

ion_source_2ion_source_3Construction techniques involve drilling, lathe turning, silver brazing, and soft silver soldering.  Photos at left show components of the source  (most prominent are the extraction electrodes) during assembly on a ConFlat cube for testing.

Extraction of ions is accomplished by a strong DC electric field imposed between the negatively-biased “nozzle” on the 5/16″ tube and the grounded septum on the 1/2″ tube.   I use up to -5 kV for extraction of ions.  The extraction nozzle also throttles neutral gas flow from the discharge region into the vacuum chamber.

am-6155_highpowerRF power is supplied by an FAA-surplus AM-6155 power amplifier operating at 200 MHz.  These amplifiers are a common hamfest bargain.  Circuit details and modifications for the amateur radio hobby are easy to find online.  To date I have not produced more than ~60W with this amplifier, driving it with signals below 1W.  Beam current depends am-6155-innardsvery strongly on RF power, and I plan to upgrade the driver for the AM-6155 to produce more.  The top photo shows this amplifier producing power (lighting a mercury-vapor discharge), and the bottom photo shows the tube compartment of the amplifier modified for shunt feed of plate current.

schemat_ltunerion_trap_3Inductive coupling of the 200-MHz power to the discharge plasma is effected with a single loop of heavy conductor that forms part of a resonant “L-match” circuit, providing an easy interface to 50-Ω cable.  This is illustrated schematically at left.  The right photo shows the ion source ready for testing, with the RF coupling loop visible along with other components including gas for the discharge (deuterium lecture bottle).

d2_is_2Photos from operation.  The top photo shows the RF deuterium discharge in a standard 19-mm (3/4″) Pyrex test tube, and below it a beam of extracted ions impinging on a graphite Faraday cup target.  RF power is about 50W, extraction voltage -3 kV, and target at -10 kV.  Background pressure has been raised into the millitorr range to enhance beam visibility.  Extracted current is 0.25 milliampere.  10kv_tBottom photo shows the exit aperture clearly, with deuteron beam passing through a ring electrode at -10 kV.  Here the extraction voltage was -5 kV.  It is not possible to accurately measure the beam current in this arrangement, but it is probably on the order of 0.5 mA.  Not surprisingly, a few neutrons from 2H(d,n) fusion reactions can be detected with higher potentials on the ring cathode.

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More information about this ion source

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Experiments with a Tiny Radioisotope Neutron Source

July 20, 2008

By placing beryllium in intimate contact with an alpha-emitting radioisotope, neutrons are produced. At home, one can approach this well-known reaction by lining up the sealed americium sources from a quantity of old-fashioned ionization smoke detectors on a sheet of beryllium metal. The neutron yield is easily detected; see this link for more information. My own toy AmBe neutron source currently produces an estimated 1000 neutrons per second. That makes it more than three orders of magnitude weaker than my Farnsworth Fusor. But are there enough neutrons to perform some detectable nuclear reactions? As it turns out…yes.

Neutrons give rise to prompt gamma rays when they are captured by many nuclei. The following experiments involve the detection of high-energy (> 4.4 MeV) gamma signatures from neutron capture in chlorine, iron, and titanium, in the form of inexpensive and readily-available compounds mixed with water and placed near the 1000 n / s AmBe source. A gamma ray spectrum is collected with a very efficient scintillation detector (2×2″ BGO) over a period of hours.

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Setup

For the chlorine experiment, a 40 lb. bag of rock salt was purchased at Lowe’s for about $4. A two-gallon pail was filled to the brim with salt, followed by successive additions of water and more salt to obtain the highest chlorine density possible. A piece of PVC pipe, capped on the bottom, enters the salt pail from the top. The neutron source fits down in this pipe so that it is nearly surrounded on all sides by salt. The lead-shielded BGO scintillator views the side of the pail at the level of the source.

The iron experiment is similar, except the medium in use is 10 lb. of ferrous oxide (FeO) purchased at New Mexico Clay here in Albuquerque. The oxide is divided among two PETE jars and water is added to wet the oxide, drive out air, and fill the jars. The neutron source is taped behind one of the jars and the assemblage is surrounded by UHMW-PE bricks to act as a neutron moderator / reflector.

Titanium was obtained in the form of titanium dioxide powder from New Mexico Clay. Enough dioxide (~8 lb) was loaded into a large cylindrical Rubbermaid food-storage container to fill the sonofabitch. Water was added to displace air and top off, and as before, PE bricks were arranged around the outside. In this instance the neutron source sits in an acrylic tube penetrating the TiO2 from the top.

Results follow below…

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Uranium Chemistry

February 20, 2008

Uranyl peroxide Uranium and its pure compounds are just not readily available to the amateur scientist, element collector, or student in 2008. So what is one to do? Make these materials oneself, of course. (At left is a quantity of home-baked yellowcake.)

This is the inaugural post in what will become a short series, detailing how uranium and various pure compounds can be refined from the brute earth to serve personal needs. There are differences between what is done in industrial mining / milling operations and what can be realistically accomplished in a typical American domicile. There are also differences in the raw materials that could be obtained back in the good old days when our favorite applied inorganic chemistry texts were written (“Borrow a gallon of fuming nitric acid and some glycerin from your science-teacher…”), versus what can be obtained in the paranoid, restrictive world of today. Thus, my approach to uranium chemistry emphasizes practical techniques and materials that are available to today’s home-dweller. The foregoing discussion assumes a decent background in chemistry and mature attention to safety.

Uranium chemicals

Uranium compounds that can be easily prepared at home are shown in this photo. In vials, left to right: uranyl oxide (UO3); uranyl peroxide (UO4·nH2O); triuranium octoxide, U3O8; sodium diuranate (Na2U2O7·6H2O); uranium tetrafluoride (UF4·2.5H2O); “sodium peruranate” in solution; uranyl chloride (UO2Cl2) in solution. In front is an electroplated layer of uranium dioxide (UO2). Click “more” below for content (I will upload it as time permits).

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