Jon Custer
  • Member for 7 years, 11 months
  • Last seen this week
  • Albuquerque NM
Why is caesium-137 more stable than caesium-134?
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34 votes

As noted in the comments, all of the various Cs isotopes I'll mention decay by emitting a beta, converting the Cs isotope to a Ba isotope. Now, while details of nuclear decays are not necessarily ...

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Is it more efficient to stack two Peltier modules or to set them side by side?
20 votes

Thermoelectric coolers (TECs) are horrendously inefficient (slightly better now than a decade ago, but not really by that much). Cooling a hunk of inert material is the easiest. Cooling and ...

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Is semiconductor theory really based on quantum mechanics?
15 votes

The question in the title is quite different from the question in the text. But, here I will refer to the Preface in Shockley's book Electrons and Holes in Semiconductors, published in 1950 by van ...

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Why is the energy spectrum of alpha decay discrete?
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9 votes

Nuclei have a series of discrete energy levels (somewhat analogous to electronic energy levels, but the details are, not surprisingly, different). Examples of these so-called Energy Level Diagrams can ...

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Could somebody explain the stress versus strain curve?
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8 votes

For question (1), the plot is actually 'normal' - a tensile test machine (e.g. Instron, no affiliation, just used them) varies the strain in the material directly, often using a giant screw, sometimes ...

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How is it we have helium on earth?
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7 votes

Alpha decay, which is why it generally comes from natural gas fields where the alphas are trapped with the methane (in the US there are several in Kansas, Oklahoma, and Texas with reasonable fractions)...

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Why do we heat salt water to desalinate it rather than freeze it?
6 votes

Looking at a phase diagram for water (this one By Materialscientist - Own work, CC BY-SA 3.0, Link) one sees that, starting below 23.3 wt% NaCl in water and above 0C you have a liquid solution (the ...

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Why absence of electron is called hole?
6 votes

In an idealized semiconductor at absolute zero, all the valence states are occupied by electrons, and all the conduction states are empty. When you take one electron an place it into the conduction ...

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What is Thermal expansion?
5 votes

If the interatomic potential were a perfect parabola, it is true that there would be no thermal expansion. Why? Well, as atoms vibrated more and more, the mean atom position would not change. Sure, it ...

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Could atmospheric muons be used to catalyze nuclear fusion?
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5 votes

OK, lets run some numbers. Assume you had a setup where every single muon actually catalyzed a D-T fusion event (note - that won't happen by a large factor). The energy released in D-T fusion is about ...

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Stacking compasses and what happens
5 votes

Two North's don't want to align next to each other - it is a lower energy state for one North end to be next to South's on the other compasses. Each compass needle is a little magnet, that aligns with ...

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What causes a eutectic point?
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5 votes

A somewhat longer answer, since I'm afraid my comment may have seemed a bit abrupt... Lets look at a fairly simple thermodynamic system, the Ag-Ge binary phase diagram. This consists of 3 phases only,...

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Why did scientists use a zinc-sulfide coated screen to detect the alpha, beta, and gamma radiation?
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5 votes

For those wondering why zinc sulfide is important, I will note that "a zinc-sulphide screen in vacuum" is specifically called out in the original Geiger and Marsden papers on alpha particle scattering....

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why stopping potential in the photoelectric effect is negative?
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5 votes

For photon energies above what is needed to kick out photoelectons, the electron is departing the surface with some excess kinetic energy (or else you'd never detect them since they'd go nowhere). A ...

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Experiment demonstrating interference patterns of neutrons
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5 votes

I would suggest 'Single- and double-slit diffraction of neutrons" by Zeilinger, Gahler, Shull, Treimer, and Mampe, Reviews of Modern Physics 60(4), 1067-1073 (1988). If I might quote the abstract: ...

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What prevents fissile materials from fully undergoing nuclear fission?
4 votes

As I noted above in the comments, Reed's paper is a wonderful overview of the conditions in a bomb, going back to Serber's Los Alamos Primer. As noted in the introduction to Reed's paper: One full ...

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What happens if you collide 2 alpha particles?
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4 votes

When looking for nuclear reactions, the place to go is the Evaluated and Compiled Nuclear Structure Data File repository of your choice. I use the one at Brookhaven. Enter in the compound nucleus (8Be)...

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How much current can be produced by Van de Graaff generators or Pelletrons?
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4 votes

Having until recently run an ion beam lab with both a High Voltage Engineering accelerator and a National Electrostatics Pelletron system, and having run a number of other similar systems over the ...

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Why are simulations like Monte Carlo or Metropolis studied for Ising Models when 1d and 2d case have analytical solutions?
4 votes

When doing computer simulations, the big question is (or should be) whether or not the computed answer has anything to do with reality. So, you research the physics, find some algorithms that seem to ...

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How can a 2.1 Hz excitation induce resonant vibrations in a structure with a fundamental frequency at 6.3 Hz?
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4 votes

Footsteps are not a pure sine wave at all. Consider them instead as more of a square wave, which clearly will have many harmonics of the fundamental frequency. Assuming a more complicated wave ...

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Why do $p$-orbitals correspond to the valence band in semiconductors?
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4 votes

There are several simplifications or confusing points in your thinking that should be clarified. (One that is not totally relevant to the discussion is that the indirect gap should, of course, be ...

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Why did they start Wendelstein 7-X With Helium?
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4 votes

Converting my comment to an answer: Well, I'd start up a new plasma system using an inert gas - I'd rather find out any problems with a non-explosive gas mixture. Once I know that everything works (...

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Why do we have heavy and light hole bands in semiconductors?
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4 votes

To delve more deeply into the origin of the various bands, you should go look at the literature where these bands are calculated. The classic reference for silicon and germanium is Energy-Band ...

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Space group setting of a crystal structure
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4 votes

Ahhh, good old crystallography causing confusion. Relax, it is not uncommon to say the least. Only long, deep study of point and space groups will lead to clarity. However, there are good resources ...

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What determines the forward voltage drop for a diode?
4 votes

Lets get something out of the way first: The threshold, or turn-on voltage, is not really an intrinsic device property per se. It originates more from a desire by circuit designers to have a rule of ...

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Carbon-14 formation in atmosphere
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4 votes

It is a prompt (immediate) reaction, and is more usually written something like N14(n,p)C14 to indicate that. It is far from the only such reaction. EDIT - To quantify my statement that there are ...

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Mott's conjecture about NiO verified or not?
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4 votes

You are in luck. Alexander Gavriliuk, Ivan Trojan, and Viktor Struzhkin observed the anticipated transition in 2012. Their paper is in Physical Review Letters 109 086402 (2012) (link to the paper). ...

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Why does water not evaporate in below 0 degrees?
4 votes

The equilibrium vapor pressure of water vapor over ice is well known and easy to google for (http://www.its.caltech.edu/~atomic/snowcrystals/ice/ice.htm is one possible link). It is slightly lower ...

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How big was the first transistor?
4 votes

From "The Transistor, A Semi-Conductor Triode", by J. Bardeen and W. H. Brattain, Phys Rev. 74(2), 230-231 (1948): "The device consists of three electrodes placed on a block of germanium as shown ...

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What does the Combined Notation $\left<\text{abc}\right>\{\text{def}\}$ mean in Crystallography?
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3 votes

Standard crystallography notation: [hkl] is a specific direction <hkl> is a family of equivalent directions ([100] and [001] are equivalent in a cubic system for example, and are <100> ...

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