Charge Sensitive Preamplifiers (CSP) for the MINIBALL Array of Detectors

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Percival Hicks
5 years ago
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1 Charge Sensitive Preamplifiers (CSP) for the MINIBALL Array of Detectors - Core & Segments CSPs for 6-fold and 12-fold segmented and encapsulated detectors; - Principle of operation, schematics, PCBs; - Adjustments, troubleshooting, maintenance.

A typical structure of a symmetric HP-Ge Detector Parameter Dielectric Electronhole pair E Mobility e / hole(+) e / hole(+) 2 [cm /Vs] Ge 16 2.

2 A typical structure of a symmetric HP-Ge Detector Parameter Dielectric Electronhole pair E Mobility e / hole(+) e / hole(+) 2 [cm /Vs] Ge [ev] 3,900 / 1,900 40,000 / 50,000 -HV (GND) + HV (~ kv/cm) C i (-) Central contact (Core) (- e ~ 10ps ) (~ mm) Standard n-type Intrinsic HP-Ge (P-I-N) Closed end Co-axial structure I(o) ~ < 10 [pa] (+) Cdet ~ 35 pf Collection time ~ ns Exterior contacts (Segments)

3 Semiconductors Detectors HP-Ge Parameter Dielectric constant o Electron-hole pair E E n [ev] Mobility e / hole(+) e / hole(+) [cm 2/Vs] Ge [ev] 3,900 / 1,900 40,000 / > 50,000 e (-) 38,500 [100]/[111] Hole (+) 61,500 [ ~ 300 o K] [ ~ 80 K]

4 Analog Nuclear Electronics Detector Signal collection Electronic Signal Processing Front-End : Preamplifier & Shaper Considerations on Detector Signal Processing Specific issues about signal processing in gamma spectroscopy: a) pulses (delta, step, ~ Gaussian form) b) they are statistically distributed (BLR) Time frequency domain signal, circuit representation, A typical analog front-end electronics, namely a detector and charge sensitive preamplifier assembly

5 Fast pipeline ADC [DGF] [HP-Ge+CSP] + an Analog or Digital Nuclear Electronics Spectroscopic Chain is used in order to extract the: E, t, position (r, azimuth) Analog Filter Amplifier Chain Fast Pipe line ADC [DGF] Collected charge pulses (+ & -) Qd -delta t UCSP exponential Pile-up of pulses UFA ~ Gaussian Baseline restorer t t Digital Filters (Fast, Slow)

6 Fast pipeline ADC [DGF] [HP-Ge+CSP] + an Analog or Digital Nuclear Electronics Spectroscopic Chain is used in order to extract the: E, t, position (r, azimuth) Analog Filter Amplifier Chain Fast pipeline ADC & [DGF] Collected charge pulses (+ & -) Qd -delta t UCSP exponential Pile-up of pulses UFA ~ Gaussian Baseline restorer t t Digital Filters (Fast, Slow)

7 Detector Signal Collection a gamma ray crossing the Ge detector generates electron-hole pairs charges are collected on electrode + - Rp Z plates (as a capacitor) building up a voltage or a current pulse Final objectives: Detector Electronic Circuit amplitude measurement (E) time measurement (t) position (radius, azimuth)

8 Detector Signal Collection + - Rp Z if Z is high, Detector charge is kept on capacitor nodes and a voltage builds up (until capacitor is discharged) Advantages: excellent energy resolution friendly pulse shape analysis position Disadvantages: channel-to-channel crosstalk pile up above 40 k c.p.s. larger sensitivity to EMI Electronic Circuit if Z is low, charge flows as a current through the impedance in a short time. Advantages: limited signal pile up (easy BLR) limited channel-to-channel crosstalk low sensitivity to EMI good time resolution Disadvantages: signal / noise ratio to low worse resolution

9 Charge Sensitive Preamplifier Active Integrator ( Charge Sensitive Preamplifier - CSP) Input impedance very high ( i.e. ~ no signal current flows into amplifier), Cf /Rf feedback capacitor /resistor between output and input, very large equivalent dynamic capacitance, sensitivity A(q) ~ - Qi / Cf, large open loop gain Ao ~ 15, ,000 clean transfer function (no ringing, no over shoots, no under shoots) -Qi Rf (Rf.Cf ~ 1ms) (tr ~ 30ns) Step function Ci ~ dynamic input capacitance - GND + Inverting A(o) jfet Ci ~ 10-20,000 pf Charge Sensitive Loop

10 Pole - Zero cancellation technique Rf. Cf ~ 1 ms Cf ~ 1pF (0.5pF-1.5pF), Rf~ 1GOhm Rd. Cd ~ 50 µs simple differentiation Rpz~ 20 k Ohm Baseline shifts Cd~ 47 nf, Rd~1.1 kohm if (Rf.Cf) = (Rpz.Cd) ~ 50 µs differentiation with P/Z adj. no baseline shifts Baseline restored

11 Parameter Sensitivity ( mv / MeV) Resolution (Cd= 0pF; cold FET) Slope ( + ev/ pf) [Cd] Rise time (Cd= 0pF); Slope ( + ns/ pf) [Cd] [50 Ohm] / Power [mw] Saturation of the 1st Open Loop Gain IKP-Cologne (Miniball jfet IF1320) ~ 175 mv/mev ( single ended ) ~ 600 ev < 10 ev / pf (cold FET) ~ 15 ns ( cold FET) ~ 0.3 ns ( ~ 25 ns / 33 pf ) ~ 4.5V /~ 450 mw ( + /- 12V Op.Amp.LM-6172) equiv. ~100 MeV (@ ~60mW_ jfet) ~ 20,000 MINIBALL Charge Sensitive Preamplifier Specifications By design optimized Transfer Function Crosstalk requirements less then 10-3 core-segment

12 dominated by voltage noise dominated by current noise Noise Bandwidth vs. Signal Bandwidth the equivalent noise charges Qn assumes a minimum when the current and voltage contributions are equal 2 2 current noise ~ (RC) voltage noise ~ 1/(RC) ~ Cd 1 / f noise ~ Cd

13 MINIBALL Charge Sensitive Preamplifier Idrain Cold Part Warm Part Buffer Inv. NonInv P/Z Adj. Output Driver 26 cm Detector Charge Sensitive Loop DC Adj. GND Cv HeKo Prototype LV Power Supply Filter

~ 10 ns / [gain x2] @33pF HeKo PSC823 tr ~ 42 ns PSC823 HeKo HeKo-Prototype (only

14 Slope ( ns/cv ) ~ 5 ns/ pf [HeKo] -Family of CSP consequences of various adjustments gain, rise time, slope, ringing HeKo tr ~ 26 ns Slope ( ns/ Driver Gain ~ 10 ns / [gain HeKo PSC823 tr ~ 42 ns PSC823 HeKo HeKo-Prototype (only non-inverting)

15 Adjustments Miniball warm CSP type HeKo NOT Connected Rf & Cf jfet Test In. I Drain (JFET) (Step 1 ) Cv - trimmer (bandwidth adj.) S1 + S3 Inverting / Non_Inverting P/Z Adj. (Step 2 ) (Helitrim. 11 turns) type of P/Z cancellation S5 DC Level Adj. (Step 3 ) N.B. by default Inverting ( R11b and R34b ON) S5

NOT Connected Rf & Cf Test In. Adjustments jfet Miniball warm CSP type PSC 823 I Drain (JFET) (Step 1.) S1 Inverting / Non_Inverting Cv - trimmer (bandwidth adj.

17 NOT Connected Rf & Cf Test In. Adjustments jfet Miniball warm CSP type PSC 823 I Drain (JFET) (Step 1.) S1 Inverting / Non_Inverting Cv - trimmer (bandwidth adj.) Inverting / Non_Inverting S2 S3 Range R(P/Z) (if NO Helitrim.!) P/Z Adj. (Step 2) Range R(P/Z) S4 (if NO Helitrim.!) Count Rate High/Low S5 DC Level Adj. (Step 3)

18 5 Step 1 PCBs & Shilding Step 4 Soldering Segments connectors Step 5 Soldering Core & 1 GOhm - HV Set of Cold HeKo mounted on Miniball Detector Step 2 Mounting PCBs and shield Step 3 Soldering Rf, Cf, jfet Technological steps in building - mounting a set of cold parts of CSPs on a 6_fold segmented encapsulated Miniball Detector

19 Miniball cold preamplifier set - new spare part (model) reworked at IKP-Köln (Jan. 2007)... and some still working cold parts but with: - bad segments shielding, - some cold solder joints, - almost braking wires

20 Miniball Cryostat & warm motherboards D1 & D3 Motherboards with CSPs clockwise rotated OK! some D2 Motherboards with CSPs counterwise rotated Wrong! - related problems PT 100 coaxial cable (x2) Metal bottom ring for BNCs

AGATA preamplifiers: issues and status

AGATA preamplifiers: issues and status Preamplifier group AGATA week Legnaro (Padova), Italy 15-19 September 2003 Speaker: Alberto Pullia, 16 September 2003 Work forces main developments Discrete hybrid