come, mr. tally mon, tally me banana

I pulled a large knife switch on the wall and heard some relays close as the bank of lights came on for the first time in months.  It was my third visit, but the place  seemed strange and out of place.  The underground concrete lined cavern forty by two hundred feet, it's low ceiling forced you to bend over even at the highest points. The geiger counter was more than half way in...

 

Gale's letter had arrived the day before.  We had been talking about carbon-14 dating.  Rather than one of his lengthy pieces it just had a single line:

Have you ever measured the radioactivity of a banana?

Well - no.  That meant I should figure that out.  Fortunately I had resources.   Although Malmstrom was an  active Air Force base with two hundred ICBMs and a SAGE computing environment, it was teen friendly if you just asked.  I had a pass and access to an unexpectedly good engineering library.  I also was physics teachers assistant during my junior year ..  I set up demonstrations and experiments, grade papers and inventory the fallout shelter.   

 

It was the height of the cold war and Great Falls was probably targeted by dozens of Soviet ICBMs.  Every Friday we'd be treated to a test of the air raid sirens just to make sure they were in working order.  A few people had private fallout shelters and some schools and businesses had public facilities.  Great Falls High could accommodate several hundred people in the reinforced tunnel.  There were medical kits, food (crackers, peanut butter, "cheese" and honey mostly), water, a diesel generator, radios, mattresses, toilets (sort of), batteries and three geiger counters.  The physics teacher was junior enough that the task of the quarterly inventory and check fell on him. In turn he handed it down to me - I was beginning to be a factotum.   

 

Now I was going to give one of the geiger counters a field test.

 

The Buttrey's on Tenth Avenue South had one of the best fruit and produce departments in town, plus my mother preferred Albertson's so I figured it would be better.  A friend and I biked over and I turned on the counter before going in so it's tubes would warm up.  I took some background readings in several locations and then moved to the bananas.  Sure enough it was much more radioactive than anywhere else.  I piled some bunches of bananas around it and wrote down the numbers.

About that time a clerk with an unpleasant look on his face approached.  I'm rather bad at lying, so I just said it's a science project.  "I'm watching you" ... he wanted us to leave.  We bought a few pounds and pedaled home.

 

Any living thing is slightly radioactive.  We're carbon based lifeforms and our body is just as happy with radioactive carbon-14 as it is with carbon-12.  In fact once a life form dies it can't add carbon-14 allowing the neat trick of carbon dating.  But there are other types of natural radioactivity.  One of the two dozen or so elements we need to survive is potassium.  It turned out an isotope - potassium-40 was the smoking gun.

 

About 93% of naturally occurring Potassium is Potassium-39 ...  19 protons and 20 neutrons in it's nucleus.  It's stable as is the second most common isotope representing almost everything else - Potassium-41.  The odd duck is Potassium-40.    It's fairly rare at about 120 parts per million, but is radioactive.¹ with three decay modes.  The most common decay mode gives off an energetic electron (beta particle) that can easily pass through a few meters of air or a few centimeters of banana. 

 

It was more of a 'is there anything interesting going on' look than a careful experiment, but it gave enough of a clue.  Bananas appeared to be several times as radioactive as other produce.²  They are rich in potassium ..  a standard 100 gram banana has a half gram of potassium and should produce about 15 decays a second. 

 

These are low numbers and nothing to worry about  - we encounter larger radioactive backgrounds all the time so there is no need to cut back on bananas, but its an interesting bit of physics.   Your body, by the way, has a enough potassium to make you slightly radioactive.  We're about 2.3 grams of potassium per kilogram of us, so a 70 kilogram person (sort of 154 pounds) gives a bit under 5,000 decays per second.  So if you didn't think you are hot...

 

And things get even more interesting when you look at the potassium's rarest decay mode.  About 0.001% of that 31 decays per gram per second produces positrons - antimatter.   Your average banana creates a bit of antimatter every few hours.  It immediately annihilates the first electron it runs into in a puff of energy³   Your body is doing this about once a minute ... you're hot as well as an antimatter generator.

 

How cool is that?

 __________

¹ It has three decay modes.  The half life is about 1.25 billion years and about 90% of the time decays into calcium-40 with an electron and antineutrino.  About ten percent of the time it decays to argon-40 (electron capture) plus a gamma and a neutrino.  (this turns out to explain the large amount of argon in the Earth's atmosphere).  Very rarely it goes to argon-40 plus a positron and a neutrino.

² Consulting my handy physics pocket handbook (don't leave home without it) I find potassium-40 is 117 ppm. A bit of fiddling around with numbers and I find one gram of natural potassium with see 31 atoms decay per second. About 27 of these will produce energetic electrons that a geiger counter can detect. The remaining three or four gammas can be detected with the geiger counter, but it''s sensitivity is low. Carbon-14 gives about an electron per gram every five seconds.

³ two gamma rays ...

 __________

You probably think I'd offer a banana recipe, but in the post holiday stupor I here's my mother's favorite salad dating back to the dawn of Jello - I don't eat it (Jello isn't vegetarian), but some people like it and it is simple 

 

Gooseberry Salad  (from the family archive)

 

One 1# can (2cups) gooseberries
2 pkgs (3oz) lemon flavored gelatin
1/2 c sugar
2 c orange juice
2 c horizontally sliced celery
1/2 c broken walnuts

Drain gooseberries, reserving syrup. Add enough water to the syrup to make 1 and 3/4 cups. Heat 1 cup of the syrup mixture just to boiling. Pour that over the gelatin and sugar, stirring to dissolve. Add the remaining 3/4 cup of the mixture and add the orange juice. Chill till partially set. Stir in gooseberries, celery, and nuts. Pour into 6x10x1 and 1/2" baking dish. Chill till firm. Makes 8 servings.

 

Comments

[Jheri]

Now I'm officially hot:)