Advanced Measurement Technology 142AH Посібник з експлуатації та обслуговування - Сторінка 16

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In this method the preamplifier and main amplifier

are set up as they would be used normally, but

with a dummy capacitor (or no capacity) on the

Input connector of the 142AH, and with the ac

voltmeter connected to the main amplifier output.

The noise voltage indicated on the meter,

designated E , is read and noted. Then a test

rms

pulse of known energy, E (in keV), is applied to

the Input and the amplitude of the resulting output

pulse, E

is measured in volts with an

out

oscilloscope. The noise spread can then be

calculated from the formula

where E

is output noise in volts on the 3400A

rms

meter, E is input signal in keV particle energy,

and E

is output signal in volts corresponding to

out

the above input. If the gain of the shaping

amplifier is adjusted so that the output pulse

height is 2.35 V for an input of 1 MeV equivalent

charge, then the rms meter will be calibrated

directly in energy (1 mV = 1 keV).

7. The noise performance of the preamplifier, as

measured by these methods, should not differ

significantly from that given in the specifications

in Section 2.

8. lf, during testing of the preamplifier and

detector, the noise performance of the

preamplifier has been verified as outlined in the

preceding section or is otherwise not suspected, a

detector may be tested to some extent by

duplicating the noise performance tests with the

detector connected in place and with normal

operating bias applied. The resulting combined

noise measurement, made either with an analyzer

or by the voltmeter method, indicates the sum in

quadrature of the separate noise sources of the

amplifier and the detector. In other words, the

total noise is given by

(N ) = (N ) + (N

tot

9. Each quantity is expressed in keV FWHM. The

quantity N

is known as the "noise width" of the

det

detector, and is included as one of the specified

parameters of each ORTEC semiconductor

detector. By use of the above equation and with a

knowledge of the noise of the preamplifier, the

noise width of the detector can be determined.

) .

det

amp

The significance of this noise width in evaluating

the detector is subject to interpretation, but

generally the actual resolution of the detector for

protons or electrons will be approximately the

same as the noise width; the resolution of the

detector for alpha particles will be poorer than the

noise width. The most useful application of

determining the noise width of a detector is in the

occasional monitoring of this quantity to verify

that the detector characteristics have not

undergone any significant change during use.

10. Use an ORTEC 419 Precision Pulse

Generator with a matched charge termination to

measure the rise time of the 142AH through the T

(timing) or E (energy) output. Connect the 419

output through the charge terminator to the Input

on the 142AH and use an oscilloscope with a fast

rise time (1 ns if possible). The rise time of the

preamplifier can then be computed by:

(Total rise time) = (Preamp rise time)

+ (Pulser rise time) + (Oscilloscope rise time) .

The rise time of the 419 is typically 3 ns.

5.2. CLEANING

If it is necessary to clean the components and/or

the printed circuit in the 142AH at any time, use

only methanol as a cleaning solvent. Do not use

compressed air or other source of pressurized gas

unless it is known to be clean and free of

compressor oil, and do not use any cleaning

agent other than methanol.

5.3. FACTORY REPAIR

This instrument can be returned to ORTEC for

service and repair at a nominal cost. Our

standard procedure for repair ensures the same

quality control and checkout that are used for a

new instrument. Always contact the Customer

Service Department at ORTEC, (865) 482-4411,

before sending in an instrument for repair to

obtain shipping instructions and so that the

required Return Authorization Number can be

assigned to the unit. Write this number on the

address label and on the package to ensure

prompt attention when it reaches the factory.