EG&G ORTEC 463 Посібник з експлуатації та обслуговування - Сторінка 16

Переглянути онлайн або завантажити pdf Посібник з експлуатації та обслуговування для Вимірювальні прилади EG&G ORTEC 463. EG&G ORTEC 463 20 сторінок. Constant fraction discriminator

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pulse from the preamplifier fast output will be less than

'V20 nsec. For such systems the 463 should be operated

in the Scint/SB mode for a time to crossover of "^2 nsec.

For surface barrier systems with the ORTEC 454 Timing

Filter Amplifier operating on the preamplifier slow output,

with a rise time of >20 nsec, the 463 should operate in the

Ge(Li) mode. The time to crossover in the Ge(Li) mode

is 'V/IS nsec; so this shaping is best for signals that originate

in most charge-sensitive preamplifiers.

Figure 7.8 is a typical timing spectrum taken with the 463

and using the system shown in Fig. 7.7 for a detector of

100-/L(m depletion depth. The timing spectrum in Fig. 7.9

was obtained with the same system for a detector with a

depletion depth of 1975 /jm. These two spectra illustrate

the usefulness of the surface barrier detector system in

a pulsed distance-measuring application. The distance reso

lution for a single laser burst with a light return equivalent

to 5.5 MeV would be about 3 in., and this intensity is

smal l compared to the output capability of such lasers.

If the time interval is averaged for several laser bursts, the

distance resolution could be reduced to only a smal l

fraction of an inch.

Detector:

•

Area • 100 mm2

•

Depth • 100 pm

463 Discriminator

Alpha Equivalent Energy of 5.48 MeV

•

2 nsec

•

FWHM = 255 psec

•

••

N-v

Channel Number

SB Timing Detector SN 10-4760

Area; 50 mm^. Bias = 400V

Depth: 1975Aim

Alpha Energy Resolution = 29.6 keV

RCA Laser Equivalent Light =

y-y

5.5 MeV

• /*.

Start Channel: Laser Pulser

/•**• i

Stop Channel: 454,463

••

Calibration: 8.7 psec/Channel

Gamma Rise Time = 15 nsec

FWHM'-' 660 psec \

.•vC

Fig. 7.8. Typical Timing Spectrum for a 100-/im Surface

Barrier Detector Using the System of Fig. 7.7.

Fig. 7.9. Typical Timing Spectrum for a 1950-/im Surface

Barrier Detector Using the System of Fig. 7.7.

7.4. TIMING WITH HIGH-RESOLUTION

LOW-ENERGY PHOTON SPECTROMETERS

A system that i llustrates a method for measuring l ifetimes

of excited states at extremely low energies is shown in

Fig. 7.10. The start channel for the ORTEC 457 Time to

Pulse Fleight Converter is a plastic scintillation detector

with an RCA—8575 photomultiplier tube and uses the 463

set for the Scint/SB

mode as the fast discriminator.

The stop channel is a 6-mm Low Energy Photon Spectrom

eter (LEPS) Ge(Li) detector operating with a fast timing

system composed of a 454 Timing Filter Amplifier and a

463 Constant Fraction Discriminator. The 454 is operated

with its Int (Integrate) not greater than 10 nsec and the

□iff (Differentiate) >100 nsec. The 454 gain should be

adjusted so that the highest energy of interest corresponds

to 'V'lO V at the 454 output. Operate the 463 in its

Ge(Li)

mode, with time to crossover about

18 nsec.

Figure 7.11 shows the timing resolution versus deposited

energy for a typical 6-mm LEPS detector. The system of

Fig. 7.10 was used to obtain the data points. A ^^Na

source is used to produce two coincident 511-keV gamma

rays from positron annihilation. The start channel detects

one of the gammas, and a Compton-scattered event in the

LEPS wil l occasional ly produce the energy selected by the

451, 455, and 416 slow channel to gate the multichannel

analyzer and permit time to be measured for that coin

cident event. By selecting the energy of interest with the