Posts Tagged ‘radiation’

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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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Videos from my recent trip to Chernobyl

September 17, 2011

Two videos from my most recent radioactive scavenger hunt in Ukraine’s Chernobyl exclusion zone are now on YouTube.  One features a pinhead-sized piece of spent nuclear fuel (pictured at left) that was carefully excavated from under about six inches of soil with the aid of a CDV-700 Geiger counter probe, taken back to our hotel through Checkpoint Lelev (where the scintillation portal monitor was conveniently out of service), and analyzed using a scintillation detector and Marek Dolleiser’s “PRA” software—a clever MCA emulator that uses one’s computer audio device as a nuclear ADC.  Check it out (I recommend selecting the HD format at the bottom of the window):

The second video illustrates some environmental radiochemistry at work, namely the affinity of the beta emitter Sr-90 for the phosphate matrix of deer antlers.  In this video I show that although the gamma activity (i.e. Cs-137 activity) in a pair of shed antlers is no different than local background, the beta activity is much higher.  The reasons for Sr-90’s notoriety are tangibly apparent: a decades-long half life that keeps it cracklin’ long after the accident, and alkaline-earth chemistry that favors uptake in bone.

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Inside Chernobyl Nuclear Power Plant 2011, Part III: Dosimetry Control Room

August 9, 2011

With a decade-plus lead on the rest of the RBMK fleet in confronting the uncertainties of the decommissioning process, which involves fuel movements and the continuing generation of radioactive wastes, Chernobyl Nuclear Power Plant must continue to remain particularly vigilant on matters of radiation safety.  There is a well-maintained underground bunker at the ABK-1 administrative building that serves as a modern emergency operations center, for example.  This year we visited the Phase I dosimetry control room as part of our tour, where a dosimetrist monitors radiation levels and aerosol levels in the rooms of the Unit 1 and Unit 2 complex, and monitors discharge of radioactivity from the operational VT-1 ventilation stack.  The dosimetry control room is accessed from the +10-meter deaerator corridor, between Unit 1 and Unit 2 reactor buildings.

Click any photo below for a larger version with my description; click again for a full-size file.

For this summer’s other photos from ChNPP, see this post and this post.

To compare and contrast facilities at Chernobyl with those at an operational RBMK-1000 plant, please see jencha’s wonderful recent photodiary from the Kursk NPP (the 12th photo shows the dosimetry control room there with obviously more modern equipment than ChNPP).

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Inside Chernobyl Nuclear Power Plant 2011, Part II: Deaerator Corridor and Unit 1 Control Room

August 8, 2011

More interior photographs from the Chernobyl Nuclear Power Plant, this selection focusing on highlights of the Unit 1 control room and the building’s perhaps most distinguishing interior feature, its 600-meter-long “Gold Corridor.” Right-click any photo and select “open in new window” (or equivalent) for a larger version with my caption.

For this summer’s photos of the Unit 3 end of the power plant, see this post.

Our photos from ChNPP last year are displayed at this site.

The floor plan below is compiled from an official plant safety document and is meant to help illustrate the geography of the power plant on the +10-meter elevation, near Units 1-2.

+10-meter floor plan, Phase I, Chernobyl Nuclear Power Plant

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Farnsworth Fusor (“Carl’s Jr.”)

February 17, 2008

This post will serve as a documentation hub for my Farnsworth fusor hobby projects. I have used these devices as neutron generators for doing activation experiments at home and at college. The fusor is a simple spherical ion source / accelerator / collider that can be built on a shoestring budget with a minimum of technical background, enabling hobbyists to access some nuclear fusion reactions. This blog post isn’t intended to provide an adequate overview of the fusor’s physics or serve as a base for technical discussion–there are already some good websites out there with those purposes. Readers with questions or interest in this technology are encouraged to engage in the discussions on the Fusor Forum.

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“Carl’s Jr.” is my current fusor project, first operated in 2006.

Star dischargeCarl’s JrCathodeFusor systemModeratorCockroft-Walton

Specifications of “Carl’s Jr.” are provided below, with links to more detail and commercial suppliers for some components.

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While a student at Guilford College, I built the predecessor to Carl’s Jr., a larger but less innovative fusor with which I did some novel neutron activation experiments.

OldFusorPoissor OldFusorTopViewxray machine sparking

Specifications of my Guilford fusor are provided below.

  • Typical operating conditions: 67 kVp, 15 mA, 10 mtorr, ~3E+06 neutrons / sec.
  • Vacuum chamber: 2 x 8″ 304SS hemispheres, 10″ equatorial CF flanges
  • Chamber ports: 2 x QF25, 1 x 2.75″ CF, 1 x special feedthrough solder lip
  • Cathode: 6 x 2.5″ loops of 0.025″ dia. 316SS wire, construction by spot welding
  • Ion source: none
  • HV feedthrough: surplus 100 kV vacuum-rated feedthrough, air-insulated
  • Gas system: 5o-liter LB cylinder of deuterium, 2 series needle valves
  • High voltage system: 135 kVp x-ray supply, controlled by Variac and magnetic amplifier
  • Vacuum system: Varian 50 LPS baby turbopump system
  • Cooling system: forced-convection air cooling (ShopVac)
  • Neutron irradiator: Water in VHS cases; paraffin canning wax

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Fusor Experiments

Documents

  • My undergraduate thesis for the physics department of Guilford College, entitled “Neutron Activation Using a Farnsworth Fusor” (2003) can be downloaded here. An accompanying PowerPoint presentation can be downloaded here.

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