Model 2101 Transistor Reset Preamplifier Option

Features • Low noise

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/ / / / TRP Never

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% I PC Energy Rate Limit \ \

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/ 80Co Input Rate (kcps)

Throughput vs. Count Rate: Throughput Optimization

• High throughput rate • Non-saturating • Good resolution vs. count rate • Pole/zero not required

Introduction The traditional preamplifier used on Ge detectors employs RC feed-back. The feedback capacitor (Cf) establishes the charge gain for the detector (V0=Qin/Cf) and the feedback resistor (Rf) discharges the feedback capacitor with a time constant of Rf x Cf. The feedback resistor effectively must conduct the total detector current which flows as a result of leakage current and photon interaction. Leakage current is normally negligible leaving detector current directly proportional to the energy rate of photon interaction. Figure 1 shows the energy rate limit imposed vs. feedback resistor value for preamplifiers having a dynamic range of 20 volts which is typical of RC preamplifiers.

If the feedback resistor value is lowered to increase the count rate limit, the noise increases and the resolution becomes worse. In addition, non-ideal behavior of components often leads to distorted pulse shapes which do not respond properly to pole-zero cancellation. This leads to resolution degradation at high count rates.

In the case of the Transistor Reset Preamplifier the basic charge sensitive preamplifier circuit remains the same but the feedback resis-tor is eliminated. The output of the preamp steps in a random staircase fashion through the dynamic range at which time a transistor switch discharges the integrator abruptly.

Refer to Figure 2 for a simplified block diagram and signal charac-teristics of the TRP.

The Transistor Reset Preamp (TRP) offers the following advantages over the RC preamplifier.

1. Lower noise (no feedback resistor) 2. Higher maximum energy rate (no saturation) 3. Better resolution vs. count rate 4. Pole-zero cancellation not required

RC Preamplifier

The energy rate limit of RC preamplifiers is a function of feedback resistor (R) and dynamic range.

Vtp = IDxR f

Assuming Vtp = 20 V, max. Energy Rate Limit (MeV/s) = 4 0 0 0 0 0 MeV/s

R (Gigohms) For 2 Gigohms - limit is 200 000 MeV/s For 0.5 Gigohms - limit is 800 000 MeV/s

Transistor Reset Preamplifier

H.V.

The comparator turns on the transistor to reset the preamp when output reaches the upper limit of its range.

Figure 2 Simplified Block Diagram

Figure 1 Preamplifier Rate Limit

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There are two disadvantages to the TRP.

• Dead time due to resets • Cost

The Canberra 2101 TRP has been carefully engineered to enhance the advantages and to minimize the disadvantages of this type of pream-plifier. The factors affecting overall performance are discussed below:

CHARGE GAIN The 2101 normally operates at 50 mV/MeV vs. 200 mV/MeV for some competitive units. This reduces dead time due to resets by 80%.

DYNAMIC RANGE The 2101 has a four volt dynamic range vs. two volts for some competitive units. This reduces dead time due to resets by 50%.

RESET TIME The 2101 has a reset time of less than two microseconds typically.

SETTLING TIME The 2101 and the companion Model 2025 amplifier have a com-bined reset recovery time of three amplifier pulse widths typically.

RANDOM RESET The 2101 reset is delayed by an arbitrary time to prevent biasing against high energy events which are more likely to initiate resets.

SYSTEM PERFORMANCE Overall performance of a detector system depends on all the components in the system - detector preamplifier, amplifier, and ADC so it is difficult to guarantee a specific system performance level. Typical results for several systems that were built and evalu-ated carefully are given below:

Parameter Results Spectrum Broadening 6% at 1.33 MeV and 100 kcps Pulses Lost to Resets 7% at 1.33 MeV and 100 kcps

ORDERING INFORMATION The 2101 is available only as an option on Canberra Coaxial and Reverse Coaxial (REGe) detectors. Two versions are offered. The 2101P is employed on Coaxial Detectors. The 2101N is employed on REGe detectors. The normal packaging matches our slimline cryostats. The 2101 is available as a special for flanged cryostats.

Specifications INPUTS

DETECTOR INPUT - Charge pulse from a cooled Ge detector. TEST INPUT - Charge coupled to preamp input at 0.5 pC/V; overall voltage gain is 0.5, nominal. H.V. INPUT - Zero to ±5000 V dc; filter time constant = 2 s, nominal; H.V. ground is isolated from signal ground by 470 Q.

OUTPUTS AND INDICATORS ENERGY OUTPUT - Step function output in random staircase fashion. Range is approximately - 2 V to +2 V dc; Z0 = 93 Q. H.V. INHIBIT OUTPUT - Logic signal disables H.V.; bias supply

when detector is warm; output is >5 V when detector is cold and <0.5 V when detector is warm. H.V. INHIBIT INDICATORS - Green LED glows when detector is cold; red LED glows when detector is warm. INHIBIT OUTPUT - Positive 5 V pulse can be used to gate ADC during and following reset; width adjustable from 2.5 to 300 us; Z0 = 50 Q.

CONTROLS RESET DELAY - Internal potentiometer delays preamp reset to allow time to finish processing the event that triggered reset; range: 1-20 us. RESET JUMPER - Enables or disables reset delay. INHIBIT WIDTH - Potentiometer controls Inhibit Pulse width over range of 2.5 to 300 [is; rear panel on box version, internal on slimline version.

CONNECTOR TYPES DETECTOR INPUT - Internal to Detector/Preamp Assembly H.V. INPUT-SHV TEST INPUT - BNC UG-1094/U ENERGY OUTPUT - BNC UG-1094/U RESET INHIBIT OUTPUT - BNC UG-1094/U WARM-UP INHIBIT OUTPUT - BNC UG-1094/U POWER - Amphenol 17-20090

POWER REQUIREMENTS + 24 V - 3 0 mA +12 V - 8 0 mA -24 V - 1 0 mA -12 V - 3 0 mA Supplied by associated main shaping amplifier.

The systems included Models GC1018 and GR1018 detectors with Model 2101 Preamplifiers, Model 2025 Amplifiers and Model 8077 ADCs. The 2025 was operated in Gaussian mode with 2 us shaping.

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