Specifications

Typical Experiment
69
TYPICAL EXPERIMENT
This experiment is typical of many photon
counting experiments. A pulsed light source is
used to pump atoms to an excited state. Decays
from the excited state are observed, allowing the
lifetime of the state to be measured.
ENERGY LEVELS OF RUBY
The energy level diagram of Ruby is shown
below. There are absorption bands around 400 nm
and 550 nm. The Cr+++ ions which absorb light at
these wavelengths decay in about 50 ns to the
upper state of the well known laser transition. This
state has a lifetime of about 3.5 ms, and decays to
the ground state by emitting a photon at 694.3 nm.
ABSORPTION
BAND
50 NS DECAY
UPPER LEVEL OF
LASER TRANSITION
SPONTANEOUS DECAY
AT 694.3 NM
WITH 3.5 MS LIFETIME
GROUND STATE
GREEN
LED
ENERGY LEVELS FOR RUBY
The absorption band at 550 nm overlaps the
emission line of a green LED. In this
demonstration experiment, a pulsed green LED is
used to quickly populate the excited state, and
decays from the excited state are seen through a
bandpass interference filter centered on Ruby's
694.3 nm emission line.
APPARATUS
The apparatus is shown below. The green LED is
glued to the edge of a 1 cm diameter, 3 mm thick,
ruby disk. The ruby disk is viewed through the
bandpass interference filter by a Hamamatsu R928
PMT. This side-on PMT was selected for its high
gain, fast rise time, and good red sensitivity.
The phototube base uses a tapered voltage divider,
with about 3x the normal interstage voltage
between the photocathode and the first dynode.
This helps to narrow the pulse height spectrum for
single photon events. The lower dynodes are
bypassed, and 100 Ohm resisters are used between
the dynodes and their bypass capacitors to reduce
ringing in the anode signal. A snubber network
consisting of a 10 inch piece of RG174 terminated
into 20 Ohms is used to further reduce anode
ringing and reduce the falltime of the output
current pulse.
OPERATION
The PMT is operated at the maximum rated high
voltage (1250 Vdc). The output pulses have an
mean amplitude of 20 mV into 50 Ohms. To
50
Av=5
1/4 SR440
SNUBBER
10" RG174
R928 PMT
694.3 NM BANDPASS
INTERFERENCE FILTER
RUBY
DISK
GREEN
LED
20Ω
TO SR400
PHOTON
COUNTER
APPARATUS FOR DEMONSTRATION EXPERIMENT