Impurities

We report here a method developed for on-line control of impurity levels in hydrogen. Using the TPC as active target we can detect the charged products of muon induced reactions with impurities. Most critical are the following admixtures: O $_{2}$ , N $_{2}$ , H $_{2}$ O and D $_{2}$ . The TPC can detect recoil nuclei (200-350keV) from $\mu$ -capture on impurities with Z $\geq$ 2 and from the $p\mu d$ fusion channel $^{3}He(0.2MeV)+\mu (5.3MeV)$ . The information is collected both with our TDC system and from 12 TPC anodes instrumented with FADC's. The selection of these events by the TPC is quite powerful, see Fig. 17. Here we report about the analysis of the FADC data, which provides detailed amplitude information, albeit with a limited data acquisition rate and volume coverage compared to the TDC's.

**Figure 17:** Capture event identified by the TPC. Signals above a third threshold at $\geq$ 100 keV where selected within a rectangular cut region after muon stop. The preliminary analysis indicates excellent signal/background ratio, where background is derived from $\mu ^{+}$ data.
$\resizebox*{0.6\textwidth}{0.4\textheight} {\includegraphics{savec.eps}}$

The $\mu$ -capture reactions are identified as events with two big signals on the muon stop anode separated in time, where the first one is the signal from stopped muon with energy up to 220 keV, the second one is from the nuclear recoil. For the selection of such event types special amplifiers and discriminators with high threshold about 70 keV and a trigger control unit was developed.

Typical transfer and capture rates as well as the calculated capture yields at 1 ppm contamination are given below. In the sequence $\mu p$ $\rightarrow \mu d$ $\rightarrow \mu$ Z capture occurs some mm away from the stop anode, because of $\mu d$ diffusion.

element	N	O
capture rate	66 $\pm$ 4	102.5 $\pm$ 1.0
transfer rate from $\mu p$ (10 $^{10}s^{-1}$ )	3.4 $\pm$ 0.7	8.5
capture yield $\mu p$ $\rightarrow \mu$ Z $\:$ @c $_{Z}$ =1 ppm	80 ppm	300 ppm
transfer rate from $\mu d$ (10 $^{10}s^{-1}$ )	14.6 $\pm$ 0.6	6.2
capture yield $\mu p$ $\rightarrow \mu d$ $\rightarrow \mu$ Z $\:$ @c $_{Z}$ =1 ppm	10 ppm	8 ppm

The main background source are $\delta$ -electrons from $\mu$ -decay electrons and double muons, i.e. two muons which can however be easily discriminated [3].

For selecting $\mu$ -capture events on impurities the following criteria were imposed on FADC events: a) Signal with amplitude larger than 50 channels on the muon stop anode, appearing during 16 $\mu s$ after the muon signal, b) the width of this signal must remain below 2 $\mu s$ , c) No other tracks in the chamber or signals on neighboring anodes at the same time.

**Figure 18:** Spectra of $\mu$ -capture events
$\resizebox*{0.6\textwidth}{0.4\textheight}{\includegraphics{muc.eps}}$

The signal spectra are shown in Fig. 18. We found 3876 $\mu$ -capture events with $\mu^-$ beam and only one event under the same requirements with $\mu^+$ beam. The number of monitored muon stops in the sensitive TPC volume was about 10

. The overall level of impurities (N

, O

and H

O) was estimated to be about 30 ppm by using chemical analysis. Based on these data we can conclude that our method has reached a sensitivity to determine impurities with Z

1 of about 0.01 ppm.