Victor Hess

Victor HessBefore making balloon ascents himself, he determined the height at which ground radiation would stop producing ionization (about 500 meters) and designed instruments that would not be damaged by temperatures and pressure changes. He then made ten ascents (five at night) - two in 1911, seven in 1912, and one in 1913 - and found that ionization soon ceased to fall off with height and began to increase rapidly, so that at a height of several miles it was many times greater than at the earth's surface. He concluded, therefore, that "a radiation of very high penetrating power enters our atmosphere from above."

On one ascent, during an almost total eclipse of the sun, the radiation was not diminished. Hess therefore concluded that the rays could not be emitted by the sun.

Hess was awarded the Nobel Prize in 1936 for this work (along with the discoverer of the Muon, Anderson).

PARTICLES!

Primary Cosmic “Rays”

Largely protons (H, hydrogen nucleus)

Energies peak ~ 300 MeV (0.3 GeV)

Come from all directions

From the galaxy, not the sun so much

~ 5000 particles /meter2/second

Only the faster ones make “good” muons ~500 protons/meter2/second

ENERGY

NU

MB

ER

Energy & eV, MeV, GeV stuff

1 eV 1 KeV 1 MeV 1 GeV 1 TeV

e p

1000 million billion trillion

Ion

ize

Ato

m

p M

ost

Po

wer

ful

Acc

eler

ato

r

e i

n T

V p

ictu

re t

ub

e C

RT

Ch

arg

ing

Sn

ail

Gam

ma

Ray

s

Vis

ible

Lig

ht

Ph

oto

n

Fas

t-is

h C

osm

ic “

Ray

s”

Mass energy

Cosmic Ray Essentialsp (cosmic “ray”)

Encounters airN or O nucleus around 15 km altitude

stuff

other stuff

Gently slows down knocking off many (~million) air molecule’s electrons

Muon may hit your sensor unless decays first

e

e

e

e

e

pionproduced

piondecays into muon

Muon Flux at Surface (above some Energy)

Muon Energy (GeV)

Mu

on

s /c

m2/s

ec

0.01

Less than1 per minuteIn a square centimeter

# Muons Change with AngleR

elat

ive

Nu

mb

er

Angle off Vertical

Muon Rate vs Altitude(ft)

Count Rate Increases ~ 100% over 10000 ft.So IF a straight line – expect 10 % increase in 1000 ft

Feet

Ct/min

How can this be?

~ 30% Difference from close locations

Muon Counting Headachesdue to Statistics

What is your best estimate of the average result if you did it a zillion times?

How confident are you with the estimate?

EXAMPLE:Count random “hits” over 30 seconds, multiple times - when the TRUE average rate is 1 per second.

One 30 Second Trial

Another 30 Second Trial

10X 30 Second Trials

Average -> TRUE with Trials

15

20

25

30

35

40

0 2 4 6 8 10 12

30 sec Trail#

HitsIn30

AveafterTrials

HitsIn30sec

TRUE

Uncertainty Reduced with Trials

15

20

25

30

35

40

45

0 2 4 6 8 10 1230 sec Trail# (accumulated)

HitsIn30sec

Best Estimate of Average # Hitsin 30 sec and an estimateof Uncertainty after 300 sec

Distribution for 1000 Trials

1000 Trials Distibution

0

20

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60

80

100

1 9 17 25 33 41 49 57 65 73 81 89 97

Hits in 30 Sec

# o

f T

ime

s f

rom

10

00

Average 30.03stdev=5.02

Is Result More for Test 1 or Test 2?

0

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0 1 2 3

0

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A Note of Caution from Professor Hess

… discovered that up to about 700 meters the ionization rate decreased but then increased with altitude …

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