Geiger-Muller Counters

Geiger-Muller Counters
ENGG 168
Fall 2009
Amir H. Golnabi
Geiger‐Müller
Counter
 
G‐M
Counter
or
simply
Geiger
tube
 
One
of
the
oldest
radia9on
detector
types
in
existence
 
Quite
old
but
s9ll
useful
 
simplicity
 
low
cost
 
ease
of
opera9on
 
Third
general
category
of
gas‐filled
detectors
a@er
ion
chambers
and
propor9onal
counters
 
Detect
ionizing
radia9on
par9cles
wikimedia.org/.../4/40/Geiger_counter.jpg
electronickits.com/.../meas/vek2645big.jpg
History
 
Johannes
(Hans)
Wilhelm
Geiger:
 
German
physicist
(1882‐1945)
 
1902:
studied
physics
and
mathema9cs
in
University
of
Erlangen.
 
1907:
began
work
with
Ernest
Rutherford
at
the
University
of
Manchester.
 
1909:
conducted
the
famous
Geiger‐Marsden
experiment
called
the
gold
foil
experiment.
Together
they
created
the
Geiger
counter.
 
1911:
discovered
the
Geiger‐Nu\all
law
(or
rule)
and
performed
experiments
that
led
to
Rutherford's
atomic
model.
 
1928:
Geiger
and
his
student
Walther
Müller
created
an
improved
version
of
the
Geiger
counter,
the
Geiger‐Müller
counter.
www.webpub.allegheny.edu
h\p://en.wikipedia.org/wiki/Hans_Geiger
low‐pressure
(~0.1
Atm)
inert
gas
such
as
helium,
neon
or
argon
metal
or
graphite
h\p://en.wikipedia.org/wiki/Geiger‐Muller_tube
G‐M
tubes
Anode
wire
~
0.1
mm
diameter
Ar+
α
500-2000V
e‐
• 
Gamma
radia9on
• 
Neutrons:
no
gas
ioniza9on
neutron‐sensi9ve
tubes:
• 
boron
or
contain
boron
trifluoride
or
helium‐3
gas
• 
neutrons
interact
with
the
boron
nuclei,
producing
alpha
par9cles
or
with
the
helium‐3
nuclei
producing
hydrogen
and
tri9um
ions
and
electrons
Low
pressure
(~0.1
Atm)
inert
gas
such
as
helium,
neon
or
argon
Window:
• 
Glass‐mantle:
beta
radia9on
and
X‐rays
(cheaper)
• 
Mica:
alpha
radia9on
(more
fragile)
Geiger
Discharge
Ioniza9on
along
the
path
of
the
primary
electron

strong
electric
field
accelerate
low
energy
electrons
towards
the
center
wire

Collisions
with
the
fill
gas
produce
excited
states
(~
11.6
eV)
that
decay
with
the
emission
of
a
UV
photon
and
electron‐ion
pairs
(~
26.4
eV
for
argon)

The
new
electrons,
plus
the
original,
are
accelerated
to
produce
a
cascade
of
ioniza9on
called
"gas
mul9plica9on"
or
a
Townsend
avalanche
(the
mul9plica9on
factor
for
one
avalanche
is
typically
106
to
108.)

Photons
emi\ed
can
either
directly
ionize
gas
molecules
or
strike
the
cathode
wall,
libera9ng
addi9onal
electrons
that
quickly
produce
addi9onal
avalanches
at
sites
removed
from
the
original

a
dense
sheath
of
ioniza9on
propagates
along
the
central
wire
in
both
direc9ons,
away
from
the
region
of
ini9al
excita9on,
producing
what
is
termed
a
Geiger‐Muller
discharge.
h\p://www.kronjaeger.com/hv‐old/radio/geiger/caltech/exp2.htm
Quenching
Posi9ve
gas
ions
that
eventually
strike
the
cathode
become
neutral
atoms
in
an
excited
Ar+
state
by
gaining
electrons
from
the
cathode.
The
excited
gas
atoms
return
to
the
ground
e‐
state
by
emi;ng
photons
and
these
photons
cause
avalanches
and
hence
spurious
pulse
discharge.
Problem:
endless
con9nuous
discharge!
Solu>on:
Quenching
• 
External:
external
circuitry
quench
the
tube
(to
remove
the
high
voltage
between
the
electrodes)

Large
R
(~108
ohms)

9me
constant
of
the
charge
collec9on
circuit
is
of
the
order
of
a
millisecond
• 
Internal:
adding
quench
gas
with
lower
ioniza9on
poten9al
and
typical
concentra9on
of
5‐10%

Charge
transfer
collisions
• 
excited
quencher
gas
ions
lose
their
energy
not
by
photon
emission
but
by
dissocia>on
into
neutral
quencher
molecules

limited
life9me
of
109
counts
• 
Halogens
(chlorine
or
bromine):
molecules
are
replenished
by
spontaneous
recombina9on
at
a
later
9me
α
Geiger
discharge
Dead
Time
Ar+
Electric
field
is
reduced
below
the
cri9cal
point
by
the
posi9ve
space
charge
α
e‐
No
gas
mul9plica9on
and
therefore,
a
second
pulse
will
not
be
observed.
~50‐100
microseconds
detector
is
inopera9ve
(dead)
for
the
9me
required
for
the
ion
sheath
to
migrate
outward
far
enough
for
the
field
gradient
to
recover
above
the
avalanche
threshold
h\p://en.wikipedia.org/wiki/Geiger‐Muller_tube
Geiger
Coun>ng
Plateau
Defini9on:
voltage
range
in
which
the
Geiger
counter
operates.
It
depends
on
the
characteris9cs
of
the
specific
counter.
• 
In
this
region
the
poten9al
difference
in
the
counter
is
strong
enough
to
ionize
all
the
gas
inside
the
tube,
upon
triggering
by
the
incoming
ionizing
radia9on
(alpha,
beta
or
gamma
radia9on).
•  Below
the
plateau
the
voltage
is
not
high
enough
to
cause
complete
discharge
• 
Star9ng
voltage
• 
Knee
~50‐100
microseconds
felix.physics.sunysb.edu/.../PHY251_Geiger.html
References
• 
G.
F.
Knoll,
Radia9on
Detec9on
and
Measurement,
1st
Edi9on
JOHN
WILEY
&
SONS
INC.
• 
The
Internet
Encyclopedia
of
Science:
Geiger‐Müller
counter,
<h\p://
www.daviddarling.info/encyclopedia/G/Geiger‐Muller_counter.html>
• 
Traveling
With
the
Atom,
Allegheny
College,
<h\p://
webpub.allegheny.edu/employee/g/grodgers/Scien9ficTravelingWebsite/
Geiger.html>
• 
The
Geiger
Counter
And
Coun9ng
Sta9s9cs,
Caltech
Senior
Physics
Laboratory,
Experiment
2,
Sep
1997.
• 
Jonathon
Nye,
Radia9on
Detec9on
and
Measurement,
Emory
University,
July
2008.
• 
Carl’s
Electronics,
GEIGER‐MULLER
COUNTER
KIT,
<h\p://
www.electronickits.com/kit/complete/meas/vek2645.htm>
• 
Wikipedia,
Hans
Geiger,
<h\p://en.wikipedia.org/wiki/Hans_Geiger>
• 
Wikipedia,
Geiger–Müller
tube,
<h\p://en.wikipedia.org/wiki/Geiger‐
Muller_tube>