### NaI Scintillators in Liquid Helium

After yesterday's attenuation calculations, I'm trying to find out if I can place the scintillator directly into the liquid helium Dewar to avoid the attenuation of flux due to the Dewar walls and solenoid windings.  I came across an interesting article that was a portion of Fernand Bedard's PhD dissertation in 1956.  It's interesting for lots of reasons.  First, Bedard was working with Hans Meissner.  Second the article mentions currents across insulating junctions with superconductors on either side in 1956, several years before Josephson would do his Nobel prize winning work on Josephson junctions.  Finally, Dr. Bedard wound up working for the NSA!

Then, there's the pragmatic stuff.  It looks like they did place their NaI crystal in the Dewar.  The article also mentions that they were able to load 2 liters of liquid helium using only 5.5 liters.  They apparently lost only 3.5 liters to boil off!  The article also has an excellent description of their procedure and of issues they had with their photomultiplier tube.  A diagram of their Dewar is shown below, (picture 1).

References:
1.  Bedard on Dewar and photomultiplier usage and of course, electrons from superconductors
http://dx.doi.org/10.1103%2FPhysRev.102.667
Bedard F., Meissner H. & Owen G. (1956). Investigation of Electron Emission from Superconductors, Physical Review, 102 (3) 667-670. DOI:

Other Handy References on NaI Detectors
Link to nice article in ROSI on NaI detectors

Increased counts down to 0 C

Temp vs. response time

### Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

### The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though!

Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in concept very simpl…

### Kids R Kapable

Just a little note to concerned ‘grownups’ everywhere.  If you look at a kid—and I mean really look—I don’t mean notice a person shorter than you, I mean make eye contact, notice their facial expression and observe their body language—If you look at a kid, don’t assume they need your help unless they’re obviously distressed, or ask for it.  You might think this is difficult call to make.  You might think, not having kids of your own, that you’re unable to make this determination.  You are.  You do in fact, already have the skills even if you’ve never been around kids  It’s a remarkably simple call to make, just use the exact same criteria you would for determining if an adult was in distress.  Because, guess what, kids and adults are in fact the same species of animal and communicate in the same way.  Honest.  If someone—adult or child—doesn’t need your help, feel free to say hello, give a wave, give a smile, but don’t—do not—try to force help on anyone that doesn’t want or need it.

Y…