Frontend Electronics

Our frontend electronics has to handle a wide dynamic range, from minimum ionizing electrons to stopping muons. Typical signals are listed in tables 3-5 below. Note that the muon signals are generally 100 times larger than the electron signals. The gas gain at our conditions is around 5000, giving around $2\cdot10 ^{5}$ electrons at the preamplifier input for a decay electron track.

Table 3: Signal characteristics of TPC and MWPC's

Parameters of physical signals at 10 bar of hydrogen	Energy(keV)	Number of $e^{-}$
Signals on TPC anodes spaced 4 mm apart
Deposited energy of stopping muons	220	$6\cdot10 ^{3}$
Energy loss of muons stopping beyond the TPC	28	750
Recoil energy for $\mu$ -capture events on impurities (e.g. N)	200-350	$(5.4-9.4)\cdot10 ^{3}$
Recoil energy for $p\mu d$ fusion products = ( $^{3}$ He)+ $\mu$	200+2.5/mm	$5.4\cdot10 ^{3}+67/$ mm
Energy loss of decay electrons	1.4	37
Signals on TPC strips (generally 25% of anode signal)
Muons stopping parallel to a single strip	600	$16\cdot10 ^{3}$
Signals on MWPC's (7 mm gap)
Minimum ionizing electron	2.4	64

Due to the high mobility of positive ions in hydrogen gas, there is a flat $\sim 10^{-3}$ tail after the large muon stop signals. This tail is a significant fraction of the smaller electron signal height. In our first runs, this tail practically destroyed the electron tracks which occurred on the same anodes right after the muon track (electron tracking was still very efficient on the non-hit anodes). To remedy this situation, a base-line restorer circuit was developed. Electrons can now be tracked efficiently everywhere in the TPC but right along the muon track.

Our front end electronics consists of Charge Sensitive Preamplifiers (CSP's) followed by Amplifier Shaping Disciminators (ASD's). The CSP's have very low noise characteristics and have an adjustable transfer function which cancels the fast components of the signal tails. The ASD's contain the threshold and trigger circuitry and prepare the digital levels for our TDC's, they also contain the base-line restorer circuitry.

Table 4: Parameters for charge sensitive preamplifiers

Parameters for CSP's	Value
Noise at $C_{input}=0$ pF	$\sigma =(500\div 600) e^{-}$
at $C_{input}=50$ pF (shaping=200ns)	$\sigma =(1600\div 1800) e^{-}$
Dynamic range (linear scale)	5000:1
Signal rise time	$\leq$ 15 ns
Signal decay time	54 ns
Sensitivity with fine tuning $\pm$ 10%	15 mV/fC = 60 mV/MeV
Cross-talk	0.2%
Feedback capacity	2 pF
Charge inject capacity for test	2 pF
Channels per standard VME card	16
Power requirements per channels	$\pm$ 5 V, I=(+12; -10) mA
Transfer function poles	1=54 ns, 1.5 $\mu s$ and 0.25 $\mu s$
Tail cancellation (no. of exponentials)	2

**Figure 5:** Circuit diagram for the chamber electronics
$\resizebox*{0.7\textwidth}{0.5\textheight}{\includegraphics{block2.eps} }$

In the test runs with the prototype TPC, the ASD's were specially matched to each of our chambers. The ones on the TPC anodes contained the base-line restorers and also a logarithmic output allowing triggering on the entire 1:5000 dynamic range. The ASD's on the MWPC's had only a low threshold on a linear output, but had faster shaping for better time resolution. The block circuit of the readout electronics is presented in Figure 5.