M. Mutterer , W.H. Trzaska , G.P. Tyurin , A.V. Evsenin , J. von Kalben , J. Kemmer , M. Kapusta ,
V.G. Lyapin , and S.V. Khlebnikov .
Institute of Nuclear Physics, University of Technology, Darmstadt, Germany
Department of Physics, University of Jyv¨askyl¨a, Finland
V.G. Khlopin Radium Institute, St. Petersburg, Russia
KETEK GmbH, Oberschleissheim, Germany
Soltan Institute for Nuclear Studies, Swierk, Poland
Identification of charged particles is an important method in nuclear spectroscopy. We have achieved a major breakthrough that makes pulse-shape discrimination (PSD) method with a single solid-state detector comparable to and sometimes better than traditional telescope technique. By using rear-side injection in over-biased surface barrier n-type Si detectors made from homogeneously doped n-TD silicon, and extracting the pulse-shape information already at the preamplifier level we have reached improved Z and even A discrimination over a wide dynamic range. Previously good separation with PSD technique required major degradation of time resolution and inferior energy resolution. Currently we have pushed down the dynamical time range to below 35 ns and reached time resolution of about 200 ps fwhm while maintaining good energy resolution characteristic of silicon detectors.
The lowest energy threshold for Z separation of intermediate mass fragments (IMF) achieved with a 250 m thick detector is equivalent to a range of about 20 m in silicon.
For
IMFs with ranges higher than 80 m of silicon we got full isotope separation. Details of this study are presented, and the application of our method in recent nuclear physics experiments is briefly discussed.
I. I NTRODUCTION
It has been known for more than 30 years that the risetime of the signal from a nuclear radiation silicon detector following the interaction of a charged particle carries the signature of the particle’s nuclear charge Z [1]. Time evolution of the current flow in the detector depends on the penetration depth (due to the difference in electron and hole mobilities) and, because of the high charge-carrier densities involved, on the delay in charge carrier separation from the highly ionized column (so-called plasma erosion time) [2],[3],[4].
Discrimination of light charged particles (LCP) or IMFs was achieved by exploiting the signal risetime from charge-sensitive preamplifiers [5], or the width of fast timing signals [3]. Recently, impressive results on the pulse-shape discrimination of IMFs were reported for rear-side injection in selected ion-implanted n-type
Si detectors, and with the bias voltage being decreased to the minimum depletion value [5],[6].
Unfortunately, such weakly biased reversed p-i-n diodes loose their good timing characteristics and energy resolution, since a full dynamical risetime range exceeding 500 ns was required for achieving good particle separation.
Furthermore, the homogeneity of the resistivity in the bulk silicon, mainly floating-zone (FZ) material, used for detector production was found to be the key parameter that limits the quality of PSD [6],[7].
The principle of particle identification by PSD in single solid-state detectors has been discussed extensively in a recent article by Pausch et al. [2]. It is demonstrated by computer simulations that the information on the mass and charge of a detected particle is encoded in the entire shape of the detector current pulse, a risetime measurement exploiting this information only partially.
The authors have also pointed out that a wide dynamical range for particle identification is obtained by injection of the particles into the rear-side (n-side) of a totally depleted detector. This method proposed first by
Ammerlaan et al. [1] takes advantage of the rising field profile with penetration depth which minimizes the plasma-erosion effect at the highest ionization density (the Bragg maximum) near the end of the track. Also, the current pulses become shorter monotonic when the particle range increases, since the slower holes have then generally to transverse a shorter distance. As a consequence, intersection of risetime-energy curves for different particles, which occurs in case of particle injection from the front-side (p-side) of the detector, is nearly avoided.
Best PSD results in a large dynamical range are expected for rear-side injection in detectors of high field strenght over their entire thickness, provided the resistivity profile of the detector material is homogeneous, and the front-end electronics has sufficient band width and good noise characteristics for probing the faster detector current flow. In the present study we have achieved a major breakthrough in
PSD resolution by utilizing over-biased detectors made from homogeneously neutron-transmutation doped (n-TD) silicon, and extracting the pulse-shape information already at the preamplifier level thus reducing electronic noise and risetime.
II. E XPERIMENTAL
M ETHOD
For practical reasons, we have fabricated surface-barrier
(SB) type silicon detectors (10x10 mm , 250 m thick) using conventional techniques, starting from 5k cm n-TD silicon wafers with both sides mirror polished.
Only soft etching was applied before vacuum evaporation of contact layers to retain the good surface characteristics. Detector currents for these ”SBn” detectors were typically 0.3
A at 150 V which corresponds to about three times the minimum bias

Figure 1: Block diagram of the electronics used for the risetime measurement of 45 V needed for full depletion. Some of these detectors fabricated could safely be operated at even higher voltages up to 250 V, with the current not exceeding 0.5
A. At a bias voltage above 100 V, the energy resolution for 5.5 MeV
-particles (typically below 20 keV fwhm) was equally good for irradiation either from the rear or the front side.
picture), with discriminator levels for the TDC start and stop indicated by cursor lines. Keeping in mind that the penetration depth of the particles is only about 20 m in silicon the difference in risetime of the fast timing signals is basically due to the different electron and hole drift times across the entire detector thickness. This difference in drift times is reflected also in the corresponding time intervals between start and stop
(denoted after as ”relative risetime”). Our method of probing the detector risetime is not capable of deducing the current flow on absolute time scale but rather to detect relative differencies.
The preamplifier unipolar timing signal is not directly applied for PSD, and is thus available for the use of the detector in a time-of-flight setup and/or for providing the necessary trigger for the data acquisition system.
The slow amplifier signal, the rising part of which is shown also in Fig. 2, has a decay time of 50 s for providing the energy information via a high-resolution spectroscopy amplifier. We have to note that the risetime of the slow preamplifier signal (being adjustable between 30 and 50 ns), which coincides with the decay time of the timing signal, limits PSD registration to an accessible time range below about the same values.
The block diagram of the electronics used in the present work is shown in Fig. 1. We have used standard commercial units except for the charge-sensitive preamplifier (model
CSTA2), and the low-noise wide-band amplifier (TFA99).
These units constructed in SMD technique were designed and built by the electronic laboratory of the Institute of Nuclear
Physics, University of Technology, Darmstadt (TUD) [8].
The CSTA2 is a dual preamplifier providing a slow energy output, and a fast timing output which is deduced from the drain of the input FET. The 2-channel fast amplifier TFA99 is a single-width NIM unit, with maximum gain of 150 and timing-filter option.
For our purpose, the standard version of the CSTA2 preamplifier was modified for providing an additional fast differentiated and 10 amplified bipolar timing signal which permits direct probing of the current flow in the detector.
From Fig.
1 it is seen that the risetime (or pulse-shape) information is deduced by splitting this fast differentiated timing signal. One part of the signal is amplified in one channel of the TFA99 and sent to a fast LeCroy 4416B discriminator set just above the noise level.
This channel provides the start signal for the TDC. The second part of the fast differentiated signal is processed in the same manner via the second TFA99 channel set to the pulse inversion mode. The negative-going part of this differentiated signal triggers again a
LeCroy 4416B discriminator, and provides the TDC stop. Our electronic setup is similar to the scheme realized previously by
England et al. [3].
Figure 2 is an illustration of the preamplifier output signals, and the amplified differentiated fast signals, taken with a
Tektronix TDS 644A oscilloscope. The example shows the case of Am particles of 5.5 MeV injected into the detector either from the front side (upper picture) or the rear side (lower
Figure 2: Preamplifier output signals for front-side (upper picture) and rear-side (lower picture) injection of 5.5 MeV -particles into the 250 m thick SBn detector, over-biased at 140 V. Fast timing signals are in channel [1], slow energy signals in [2], and the non-inverted (inverted) and amplified fast differentiated signals for probing the detector risetime in channel [3] ([4]). Time scale is 10 ns/division. Pulses shown are averaged over 10 events. Settings for the fast discriminator levels are indicated by cursor lines.

III. R ESULTS
The PSD capability with these detectors was tested at the K=130 cyclotron facility of the University of Jyv¨askyl¨a,
Finland, with IMFs produced at a scattering angle of about in the reaction of a 150 MeV N beam on a C target.
Detectors were operated in high vacuum, at an ambient temperature of 23 C.
214 ps resolution (fwhm) for the relative risetime, for N ions at 120 MeV as an example. We also note that all SBn detectors produced so far have performed equally well.
Figure 3: Example of pulse-shape based particle identification obtained with improved fast electronics and a SBn detector made from
250 m thick n-TD silicon. The detector is over-biased at 140 V.
Energy projection of the 2D picture is shown below. The sharp peak at
150 MeV comes from N scattering on trace amounts of lead in the carbon target. Other peaks are broadened due to kinematics. The time axis in the 2D plot reflects the widths of the current pulses including electronic rise times. For IMFs lighter than N ions, particles with highest energies penetrate the detector.
Our approach has turned out to be successful as demonstrated in Fig.
3.
We have reduced the required dynamical time range of PSD for all measured IMFs down to below 35 ns, so avoiding any energy distortion by the ballistic deficit. The Z-discrimination threshold for the IMFs (dZ =1) is at a rather low energy level equivalent to about 20 m range in silicon making it a very attractive alternative for traditional dE-E telescope technique.
Individual isotopes (dA=1) are resolved at the higher energies, with an energy threshold equivalent to about 50 m range for Li and Be ions, and to about 80 m for C to O ions. With the newly fabricated SBn detectors and the improved fast electronics, we have reached
Figure 4: dE-E plot obtained with the 250 m thick SBn detector operated as dE and a 380 m thick ion-implanted detector as E.
Particles come from a C target irradiated with 150 MeV Ni beam.
Top: high amplification of the dE signal. Bottom: low amplification of the dE signal. At this beam energy ions heavier than carbon could not penetrate the dE detector, while H, He and Li isotopes with highest energies penetrate the entire thickness of the telescope.
Thanks to their homogeneity and surface characteristics, the SBn detectors make also very good dE detectors for particles that can penetrate the 250 m thickness, which is the case for IMFs lighter than N ions.
For discrimination of these high-energy particles a commercially available ion-implanted silicon detector (Type: SFH871 fabricated by
SIEMENS, M¨unchen) of 380 m thickness was positioned behind the SBn. Figure 4 is a display of the dE-E response for this telescope device, with two different settings for the amplification in the SBn dE channel. The quality achieved for particle discrimination proves the high homogeneity and energy resolution of the SBn detector.
Combining the results from Figs. 3 and 4 demonstrates that proper PSD in suitable telescope front detectors is well suited for lowering considerably the energy threshold for particle discrimination.
Very recently, SBn detectors were tested with heavier particles produced in the reaction of a 400 MeV Ne beam on a mixed Al + Au target. Increasing bias voltage to 240 V has permitted to reduce the required dynamical time range of
PSD even for reaction products as heavy as fission fragment

down to below 25 ns, as is seen from Fig. 5. In that case the resolution for the lightest IMFs was observed to become worse which, however, is partly attributed to the interference with the high flux of -electrons ejected from the target by the energetic beam. Nevertheless, this mode of operation may prove to be important as it allows to separate fission fragments (FF) from the lighter ions including IMFs.
injection, problems with particle discrimination at low energies, and noticeably more events that fall in-between the well-defined ridges. We attribute these effects to a relatively poor quality of the rear surfaces, and the use of less homogeneous FZ silicon for their production.
Already now our PSD method for particle identification is being used for highly demanding spectroscopical purposes, such as measurements of rainbow scattering carried out at the
Department of Physics, Jyv¨askyl¨a, Finland, and an experiment on light-charged-particle-accompanied fission performed at the Institute Laue-Langevin, Grenoble, France. In the latter experiment [9], our PSD electronics was applied for the first time to a larger number of silicon detectors, taking advantage of the fact that besides of the modified preamplifiers and fast amplifiers only standard commercial units are needed.
For this experiment, two arrays with 24 commercially available larger-size ion-implanted silicon detectors (Type: SFH873 fabricated by SIEMENS, M¨unchen) of 30 30 mm area and
380 m thickness were mounted on transmission frames for read-side injection.
With operating these detectors above depletion voltage full separation of - particles from ternary fission (energies up to 25 MeV) and ternary hydrogen isotopes was achieved, while retaining the good energy resolution required in the experiment.
Figure 5: Example of pulse-shape based particle identification obtained with a 250 m thick SBn detector, over-biased at 240 V.
Particles come from a mixed Al + Au target irradiated with 400
MeV Ne beam. The reaction with the gold target produces fission fragments (FF) with atomic numbers Z 44.
V. A
CKNOWLEDGEMENTS
We are indebted to U. Bonnes and the electronic group of the Institute of Nuclear Physics of TUD for their close collaboration in adapting the front-end electronics according to our needs, to the accelerator staff of the University of
Jyv¨askyl¨a for efficiently providing the beams, and to M.
Moszynski for valuable discussions. Financial support of the
Academy of Finland, Center for International Mobility, and the Nordic Council of Ministers for Research in Finland is gratefully acknowledged. This work has been supported in part by the Access to Large Facility program under the Training and
Mobility of Researchers program of the European Union.
IV. D
ISCUSSION
Our results have demonstrated that high homogeneity of the silicon material, good quality of both surfaces of the detector, and fast low-noise electronics are required for good particle identification without compromising the energy performance of the detector. Further work is in progress to understand better processes happening during the creation and collection of charges in the detector, in particular the influence of plasma erosion on PSD resolution. Special attention will be paid also to the energy threshold of our method, and to the application of SBn type detectors in time-of-flight measurements of low-energy LCPs and IMFs.
With the present electronics we have also tested several other detectors, including Si(Li) detectors of 1 to 3 mm thickness, commercially produced ion-implanted detectors (100 and 900 mm area , 380 m thick), and a conventional 1.1 mm thick SB detector. They all show useful particle separation for the H and
He isotopes, but considerably worse performance for the IMFs.
The ion-implanted detectors - the best performers among the other detectors - exhibit degraded energy resolution for rear-side
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