Front Panel
1. Introduction
The Canberra Model 7401 is a complete alpha spectrometer that includes a vacuum
chamber, bias supply, preamp/amplifier, pulser, discriminator, counter, and digital
display in a versatile double-width NIM. Samples up to two inches in diameter can
be analyzed in the chamber, which is made of stainless steel for low background and
ease of cleaning. The chamber will accommodate most charged particle detectors,
including Canberra’s rugged, low background, high resolution PIPS Detectors with
active areas up to 1200 mm2. A stainless steel shelf and sample holder are provided,
with reproducible detector/source spacing selectable from 1 to 49 mm, graduated in 4
mm increments.
The functionality of the 7401 is enhanced by use of a microprocessor-managed user
interface. Front panel controls allow the user to set and read high voltage bias,
calibration pulser energy, and discriminator and preset time settings for the built-in
counter. Additionally, the front panel display provides readout of pressure, detector
leakage current, counts and elapsed time. When the 7401 is switched off, the high
voltage, discriminator and pulser settings are retained in memory. For gross alpha
counting, the counter can be started, stopped and reset using the front panel switches,
and time readout can be switched from seconds to hours.
The built-in detector bias supply is adjustable to ±198 V dc. To avoid premature
application of bias to the detector, a switchable interlock inhibits the bias supply until
proper pressure is reached, then the high voltage is slowly increased to its
predetermined setting in approximately one minute.
The preamp and amplifier provide optimal pulse shaping and signal conditioning for
detectors used in Alpha Spectroscopy. Gain and offset of the amplifier can be
adjusted by means of front panel multi-turn controls. The test pulser can be set to
mark energies from 0.1 to 10 MeV and can be used to calibrate the spectroscopy
system without the use of alpha sources. The calibrated counter discriminator can
also be set between 0.1 and 10 MeV and an auxiliary discriminator output is
available on the rear panel. An external test pulser can be connected to the rear of the
unit. An MCA can easily accommodate multiple 7401 Alpha Spectrometers using
suitable Canberra Mixer/Routers and Multiplexers.
Chamber pressure is manually controlled by a three position valve. The stainless steel
door is equipped with hinges which automatically adjust for proper gasket
compression and the vacuum chamber is helium leak tested to 10-10 cc/s. The 7401
has a pressure gauge with a range of 0-1000 µm Hg (0-133 Pa).
Reverse sample bias is available as an option for the 7401 as Model 7401-RSB. The
reverse bias is provided by four 3-volt batteries. For those who need to operate at a
decreased vacuum, the Model 7401VR Alpha Spectrometer has a pressure gauge
range of 0-20 mm Hg (0-2.67 kPa) and includes the 7401-RSB Reverse Sample Bias
option.
7401/7401VR Alpha Spectrometer User’s Manual - 9231603C 1
