In-Beam Diamond Start Detectors

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 58, NO. 4, AUGUST 2011 2073

In-Beam Diamond Start DetectorsM. Ciobanu, E. Berdermann, N. Herrmann, K. D. Hildenbrand, M. Kis, W. Koenig, J. Pietraszko, M. Pomorski,

M. Rebisz-Pomorska, and A. Schüttauf

Abstract—This paper describes operation principles and thein-beam performance of Start Detector (SD) assemblies consistingof Diamond Detectors (DDs) grown by Chemical Vapour Depo-sition (CVD) and Front End Electronics (FEE) which have beendesigned for and used in various nuclear physics experiments atGSI Helmholtz Center for Heavy Ion Research in Darmstadt.

In parallel to the FEE design we have performed extensive calcu-lations to model the dependence of the signal-to-noise ratio � �and the time resolution on various quantities such as the col-lected charge , the detector capacitance , the temperature

, and finally the noise contribution and bandwidth of the ampli-fier.

In combination with the new FEEs (including an application-specific integrated circuit, ASIC) we have tested both polycrys-talline and single-crystal diamonds of various sizes and thicknesseswith relativistic ion beams ranging from protons to heaviest ions.

For heavy ions all setups deliver time resolutions �� ps. Incase of protons the small primary detector signals require single-crystals as material and more elaborated designs like segmenta-tion of the detector area and the increase of the amplifier inputimpedance. The best time resolution obtained for relativistic pro-tons was � �� ps.

Index Terms—Analog integrated circuits, detector instrumenta-tion, timing circuits, timing electronics.

I. INTRODUCTION

A N in in-beam Start Detector (SD) is an important com-ponent of Time-of-Flight (TOF) systems, which are com-

monly used to identify different reaction products in fixed-targetnuclear physics experiments. It provides the time-zero orreference time for the TOF measurement by determining the

Manuscript received September 24, 2010; revised February 10, 2011 andApril 17, 2011; accepted June 05, 2011. Date of publication July 25, 2011; dateof current version August 17, 2011. This work was supported in part by the Eu-ropean Community through the FP6 Integrated Infrastructure Initiative HadronPhysics, Proj. RII3-CT-2004-506078 and in part by the German BMBF underprojects 06HD154 and 06HD190i.

M. Ciobanu is with the GSI Helmholtz Zentrum für SchwerionenforschungGmbH, 64291 Darmstadt, Germany. He is also with the Space Sciences Insti-tute, Bucharest 077125, Romania (e-mail: [email protected]).

E. Berdermann, K. D. Hildenbrand, W. Koenig and A. Schüttauf arewith the GSI Helmholtz Zentrum für Schwerionenforschung GmbH, 64291Darmstadt, Germany (e-mail: [email protected]; [email protected];[email protected]; [email protected]).

N. Herrmann is with the Physikalisches Institut der Universität Heidelberg,Heidelberg 69120, Germany (e-mail: [email protected]).

M. Kis is with the GSI Helmholtz Zentrum für Schwerionenforschung GmbH,64291 Darmstadt, Germany. He is also with the Rud-er Boskoviá Institute, 10000Zagreb, Croatia (e-mail: [email protected]).

J. Pietraszko is with the Institut für Kernphysik der Johann Wolfgang GoetheUniversität Frankfurt, 60438 Frankfurt, Germany (e-mail: [email protected]).

M. Pomorski and M. Rebisz-Pomorska are with the Diamond Sensor Lab-oratory of CEA-LIST Saclay, F-91191 Gif-sur-Yvette, France (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TNS.2011.2160282

crossing time of each beam particle before it hits the target. Sothe uncertainty in affects directly the TOF resolution andhence the velocity and the mass identification [1]. In order toreach TOF resolutions below 100 ps, the time resolution ofthe SD must be below the half of that value. In addition, an-other feature may be of importance: high beam intensities re-quire fast detectors delivering narrow signals and electronicswhich can cope with the corresponding rates. In connection withthis the material should be radiation hard to withstand high in-tegral doses without sizeable degradation of its properties.

In the next section we describe briefly the physical parametersof diamonds, which make them an almost ideal active detectormaterial to meet the mentioned requirements.

A prerequisite, however, is an optimal adaptation of the usedFEE. In order to understand better the underlying relations anddependencies of parameters, such as the signal-to-noise ratio

and the time resolution on various system parameters,we have performed model calculations the results of which haveinfluenced especially the design of the detectors for minimum-ionizing protons.

Formalism and the results are described in Section III. Westart with the simplest model of readout electronics (just oneresistor) to evaluate the dependence of and on the col-lected charge , on the detector capacitance and on thetemperature . In a few steps we add the contribution of theamplifier noise and its bandwidth, resulting in a set of analyticalformulae describing the dependence of and on these pa-rameters. Two extreme cases are discussed: The hypotheticallyinstantaneous charge collection and the more realistic case ofcharge drift to the opposite electrodes. In Section IV we presentseveral SD-setups, which have been designed for different ex-periments, and their resolutions which have been achievedwith various beams of relativistic ions.

All diamonds used in the present studies have been grownat Element Six Ltd. (King’s Ride Park, Ascot, BerkshireSL58BP, UK) and supplied by DIAMOND DETECTORSLTD (16 Fleetsbridge Business Centre, Upton Road, Ponte,Dorset BH17AF, UK). We investigated both types: polycrys-talline diamond detectors (pcDD) grown on silicon wafers andsingle-crystal diamond detectors (scDD) grown on mono-crys-talline {100}-oriented High-Pressure-High-Temperature(HPHT) synthetic diamond substrates.

II. CVD DIAMONDS AS DETECTOR MATERIAL

In comparison to any other known detector material, dia-mond features a high mobility for both carriers, which canbe derived from the field-dependent carrier drift velocities

measured with broadband amplifiers (BBA) applyingthe Transient Current Technique (TCT) with short-range

-particles [2]. Note that the FWHM of the TCT signals

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2074 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 58, NO. 4, AUGUST 2011

represents the carriers transition time withthe detector thickness , whereas the signal area correspondsto . The directly measured mobility values obtained byone of the authors for scDD at lowest possible electric fields

are cm and cm atm and m, respectively [3].

These mobilities in conjunction with very high breakdownfields and a low dielectric constant

provide detector signals of fast rise times and narrowpulse widths suitable for fast timing and counting of high ionrates. Moreover, diamond is radiation hard [4], most likelydue to the high atomic packing factor of the carbon atoms inthe diamond lattice causing strong covalent bonds. The sameorigin compels a wide bandgap eV , whichprovides diamond sensors of very low dark current but also ofhigh pair production energy eV, and thus of rela-tively low sensitivity, e.g., compared to silicon eV .Its thermal conductivity (20 ), which is six timeshigher than that of copper, supports self-annealing of shallowtraps and protects in-beam detectors against thermal spikes.

In scDDs, practically 100% of the generated primary chargeis collected. In pcDD a part of this charge is lost at grain bound-aries resulting in reduced charge collection efficiencies

between 0.1 and 0.6 (with the ion generatedcharge). Due to the current production technology however, thesize of standard scDDs do not exceed 5 mm 5 mm, which isgiven by the dimensions of the HPHT diamond substrates usedfor the epitaxial growth. The maximum area available so far mayreach 10 mm 10 mm.

For precise calculations of the amplitudes of signals emittedby relativistic heavy ions in diamond sensors of different quality,the transient current signal shape has to be known; such simula-tions are not available at present. Hence, we estimated roughlythe pulse heights delivered by two diamond detectors (DD) of300 m thickness on the 50 input impedance of a BBA, takeninto account signal widths (FWHM) of 2.5 ns and 1.5 ns for ascDD and a pcDD , respectively [5].

The results plotted in Fig. 1 have been calculated for particlesof 1 A GeV kinetic energy. The horizontal solid line indicatesthe equivalent noise amplitude of the diamond broadband am-plifier (DBA) [6], i.e., , whereas the dashedline the deviation noise amplitude. Fig. 1 illustratesthe large dynamic range of signals expected in hadron physicsexperiments. Furthermore, it shows that the BBA readout in thecase of relativistic ions lighter than carbon is challenging evenfor spectroscopic grade single-crystal diamond sensors.

III. HOW CAN THE DETECTION OF RELATIVISTIC PROTONS

BE IMPROVED?

Fig. 2 shows the two FEE schematics, which are most fre-quently used to read out DDs: (a) one DD electrode is connectedto ground; the second one is biased and read out, (b) one elec-trode is biased, the second one read out. The bias is applied bythe power supply via resistor . In mode (b), the capacitor

decouples the biasing resistor thus eliminating its thermalnoise. Due to the high volume resistivity of the material, the

Fig. 1. Estimated pulse heights on the input of a BBA. The data have beencalculated for various relativistic heavy ions of a kinetic energy of 1 A GeVtraversing two DDs of � � �� m and �� and 1.0, respectively.

breakdown voltage is very high and the leakage current at nom-inal operation bias is negligible. The equivalent schematic com-prises a pure capacitor and a current generator.

A. Signal Processing at Instantaneous Collection of Charge

In the following we will calculate the S/N using the simplifiedschematics shown in Fig. 3. The diamond detector is connectedto the simplest measurement system represented by the resistor

. This approach is similar to: “S/N versus Capacitance in aCurrent-Sensing Amplifier” [7], [8] and we will continue withthe evaluation of the time resolution .

This schematics represents a low pass filter with a time con-stant and a cut-off frequency . Forsimplicity it is assumed that the induced current is a delta pulse

carrying the total collected charge ,

whereas the current discharging of the capacitor is given by

(1)

The charge conservation law predicts,

so that

(2)

The peak current at , corresponds to the signal

(3)

so that

(4)

CIOBANU et al.: IN-BEAM DIAMOND START DETECTORS 2075

Fig. 2. Schematic layout of two circuits for the connection of a DD to its amplifier (see text for explanation).

Fig. 3. Simplified equivalent schematic for noise estimations: the detector(dashed box) is connected to a simple measurement device represented by theresistor � .

Equation (4) is the amplitude response to the input delta pulse. The noise power generated in a passive resistor de-

pends on temperature and bandwidth :

with the Boltzmann constant , being the thermal currentnoise generator describing the noise generated in

(5)

For the evaluation of the noise voltage we take into ac-count the current density and the complex impedance

, which results from the parallel connection of and :

By the integration from to we obtain the total noise ofthe system given in (6)

(6)

From (3) and (6) we obtain the ratio:

(7)

In the simplified approach of Fig. 3, the depends onlyon , and . In the derivation the signal voltagepeak is assumed at . However, in real time measure-ments we have to consider the time when the signal crosses thediscriminator threshold. The charge generation in a DD is muchfaster than the time constant of any actual amplifier. The lim-ited rise time of the amplifier affects the time resolution[9]–[11]:

(8)

where is the noise dispersion and the signal slope (SL)at the discrimination level. The maximum slope at the amplifier

output can be estimated to , whereis the amplifier rise time (10% to 90% amplitude level). For anamplifier with a single pole and the frequency bandwidthone obtains and

. Hence, we can rewrite (8) as follows:

(9)

It has been assumed that the noise dispersion is equal to thenoise voltage estimated in (6). Concluding, the simpleschematic of Fig. 3 allows the estimation of the maximum per-formance that can be reached in terms of or . It alsooutlines the importance of the reduction of the total capacitanceincluding the diamond capacitance, the amplifier input capaci-tance and parasitic capacitances of the setup.

This discussion can be extended to a more accurate definitionof the noise contribution of a BBA connected to a DD. In RFapplications, the noise factor parameter is used to describethe ratio of the total noise at amplifier output related to the noisecontribution of the source resistor :

(10)

(Note that data sheets normally use the Noise Figure parameter, which is related to by (dB)). We can

estimate the noise contribution of the amplifier according to

or translated in noise current values, as given in (11)

(11)

If we can consider in Fig. 3 as a noiseless resistor we canmodify (5) to obtain , i.e., the noise generator which repre-sents the total noise contribution of the amplifier:

(12)

and the final expressions for the (7) and (9) are

(13)

and

(14)

As expected, the use of low noise amplifiers is mandatory forboth good ratio and time resolution.


In (13) the “shaping” effect of the BBA is not specified,whereas in (14) only the slope limitation is estimated. In thefollowing we will try a more accurate approach. Consideringthat the BBA has the low frequency gain and the gain-fre-quency dependence of a single pole of cut-off frequency

,

(15)

The noise voltage at the amplifier output is

(16)

By integration of (16) from to we obtain the total noiseat the amplifier output (17)

(17)

where . The response of the amplifier to thedelta function excitation is

(18)

with

(19)

For the evaluation of the output signal, we have to use the con-volution of the signal developed on (3) withgiven in (18):

a) for the particular case of

(20)

has a maximum for

b) for

(21)

is maximal for

(22)

Fig. 4. �� (left) and � (right) as a function of � with � as parameter.

If we can measure the peak value by peak detectionwe find

(23)

(24)

We can calculate the ratio;a) for the particular case

(25)

b) for

(26)In order to obtain we must calculate the slope of

in the discrimination point, e.g., for simplicity at :

For :

(27)

Equations (26) and (27) represent the ideal case of a Dirac ex-citation where the charge is instantaneously collected. In Fig. 4,

(26) and (27) are plotted as a function of for threevalues of (50 , 500 , and 5 ). In this evaluation the fol-lowing parameters have been used: , ps,

, .The increases continuously with decreasing detector ca-

pacitance. The influence of the value is minimal forand vanishes for . The ballistic

deficit of the preamplifier generates an important loss of thesignal amplitude for , which can becompensated by an increase of , leading to an increase of


Fig. 5. �� (left) and � (right) as a function of � with � as parameter.

Fig. 6. Simulated signals of a DD with � � �� . Full symbols denotethe detector output signals (left ordinate), open symbols the corresponding BBAoutput signals (right ordinate). Three different values of � are considered (50�, 500�, and 5 ��) corresponding to � � � ps, 50 ps and 500 ps, respectively.

. The time resolution degrades linearly with increasingdetector capacitance and increasing value.

For the calculations of the dependence of and on(Fig. 5), the following parameters have been used: ,

, , . Both and improvesignificantly for low capacitances and low values. The band-width of the low noise amplifier needed is technologically lim-ited to a few GHz at present (e.g., , corre-sponding to a time constant of ps). Therefore, the mosteffective way for improvement is a decrease of the total capaci-tance.

Figs. 6, 7 and 8 show results of simulations with theAPLAC simulation engine [12]. Detectors of capacitances

, 1 pF and 10 pF, respectively, have been as-sumed, all readout with BBA (Fig. 2 left). The transient signalson the detector ( , left scale) are represented by full symbols,those at the amplifier output ( , right scale) by open symbols;a very fast injected charge has been assumed (2 fC in 0.2ps). The time axis is in logarithmic scale. In each figure threevalues of are plotted: 50 (squares), 500 (circles) and5 (triangles). In all cases, ps. The maximumsignal amplitude is obtained for minimum detector capacitance

(Fig. 6). Nevertheless, it is obvious that allcases show ballistic deficits (note the different Y-scales andremember the amplifier gain ).

Fig. 7. Same as Fig. 6 but for � � � �� and � � �� ps, 500 ps and 5 ns.

Fig. 8. Same as Fig. 6 but for � � �� and � � �� ps, 5 ns and 50 ns.

The minimum ballistic deficit is observed for(Fig. 8), but the signals are very small in this case. The designvalues of and have to be a compromise between the mosteffective charge collection, the ballistic deficit and the high ratecapability of the sensors.

B. Signal Processing at Complete Charge Drift and Collection

According to the Shockley-Ramo theorem, charge movementinduces a time dependent current on the detector electrodes. Forparallel plate detector geometry and under the assumption ofa homogeneously distributed space charge within a scDD, theinduced current can be described by the simplified formula (28)[2], [3]:

(28)

where is the generated charge, is the charge car-rier velocity, is the detector thickness, denotes the lifetimeof excess electrons and holes and is given by

(29)


denotes the net effective fixed space charge in the diamondbulk, the diamond permittivity, the vacuum permittivity and

the elementary charge. If a negligible amount of space chargeis present, the internal electric field is almost constant. If thecharge carrier lifetime is longer then the transition time ,the carrier drift velocity can be calculated by . Inthis case, a constant induced current is expected for :

Equations (20) and (21) represent the response of the wholesystem to the pulse ; in order to obtain the responseto we must normalize

(30)

To obtain the amplifier response to , we use theconvolution

For a)

(31)

and b)

(32)

Fig. 9 shows the signal response at the output of a BBA (32)for a diamond detector of (squares), 1 pF (dots)and 10 pF (triangles), respectively. The upper-left figure is cal-culated for an , the upper-right for andthe bottom one for .

We can estimate the signal amplitude as well as andfor

(33)

This formula has a maximum for (22) when ,

(34)

Fig. 9. Transient response of a scDD with � � � ns and three values of �(0.1 pF, 1 pF and 10 pF), for� � �� (upper-left),� � �� (upper-right)and � � � �� (bottom).

Fig. 10. �� ratio (left) and � (right) as a function of� for three� values.

We use (33) and (34) to evaluate the most difficult case, i.e.,close to the detection of relativistic protons. The following pa-rameters have been used in Fig. 10: single crystal with

ns, , ps, , , ns.The is less than 1 for any if (left). Theincrease of to 500 allows a forand shows a maximum for , when

ns . In the case of , an optimum situationcan be reached for when ns .

Concluding, the has an optimum for i.e., whenthe detector time constant equals the carrier drift time.

has also a strong dependence on , hence the decrease ofthe detector capacitance is mandatory. Fig. 11 demonstrates thatin the case of there is no way to obtain a reasonable

and for fast protons by selecting values for and .

IV. APPLICATIONS

Over the past years we have designed and constructed anumber of SDs for various experiments at GSI. The presentmaterial is meant as a comprehensive description of the workwe have performed. Some parts may have been presentedelsewhere, in particular details of our FEE designs: these casesare properly referenced.

In the following, we give a short overview of these designsand the test results obtained with relativistic beams ranging from


Fig. 11. �� ratio (left) and � (right) as a function of � , for three values of� .

protons to tantalum ions. The results of the model simulationspresented above served as a guideline for the various projects,especially for the design of proton detectors.

A new diamond SD for the FOPI (FOur PI) detector [13],was planned to replace the standard plastic scintillation startdetector. At that time only pcDDs were available in the nec-essary size; for readout, discrete amplifiers (FEE-1-cards [11])were used. The results were more than encouraging; excellenttime resolutions have been obtained. Details are described inSection IV-A.

Hence, we began to design an even larger system, whichincluded also beam-halo detectors surrounding the central SD(Section IV-B). The influence of all relevant capacitances to theresolution was demonstrated in the model results; so we tried toreduce the external capacitances by replacing the discrete FEEthrough the newly developed ASIC PADI-1. It houses threechannels designed in a spirit very close to that of the discreteFEE-1 amplifiers [14]. This allowed placing the diamondSD directly on a printed-circuit board, which carries also theamplifier and discrimination system in very close environment.

Considering the model results and the resolutions reachedwith heavy ions it was obvious that decent results for protonswould clearly need single-crystal instead of polycrystallinematerial. The available areas of about 22 mm would presentalready too large capacitance, so the surface had to be seg-mented and the amplifier had to be placed extremely close tothe electrodes. Different technical approaches are described inSections IV-C and IV-D; this last design was developed to beused in the HADES experiment [15] at GSI.

The time resolutions of the various DD/FEE assemblies havealways been measured in the following way: two identical SDswere placed one behind another in the direct beam; in a spe-cial experiment the second sensor was replaced by a fast scin-tillation counter. The generalized setup is shown in Fig. 12.The signals of the two SDs were processed by FEE, which washousing an amplifier (A) followed by a leading edge discrimi-nator (D). The OR signals provide the common start signal ofthe Time-to-Digital Converters (TD1, TD2) and two passivelydelayed signals (DEL) stop the individual TD channels. Thetime-difference spectrum (i.e., the histogram of the time differ-ence can be approximated with anormal distribution of standard deviation . Assuming equalcontributions of the two identical SDs, the time resolution ofone channel is given by .

Since the total capacitance of the setup is of high importance,we discuss in the following in more detail. The capacitance

Fig. 12. Circuit for the measurement of the time resolution of SDs. Two iden-tical detector channels are used (see text).

of a round disk capacitor of radius , thickness , and dielectricconstant can be calculated according to [16]:

(35)

The first term represents the bulk capacitance defined only bythe two circular electrodes; the fringe fields are described by theother terms and account for up to 20% of . For other shapesand geometries, the share of the fringe capacitance may be evenhigher; it can be estimated on the basis of approximating for-mulae or by simulation programs. We decided to measure thetotal detector capacitance ; we used Boonton CapacitanceMeter Model 72BD. In the following, we will specify bytwo values: the estimated value , which ignores the fringeeffects and the measured value , which is the total valueincluding all assembly capacitances: the detector bulk capaci-tance plus fringe as well as the parasitic and the preamplifierinput capacitances (not biased) for the FEE located close to theDD.

A. Start Detectors for Heavy Ions

As a new in-beam SD for the FOPI experiment, a pcDD hasbeen mounted in the center of an aluminum box (Fig. 13, leftphoto); metallized mylar entrance and exit windows close thebox. The crystal has an area of 10 mm 10 mm, a thicknessof 0.5 mm, and it is fixed on a ceramic substrate. Each sur-face is covered by a round Cr/Au-electrode of 8 mm diameter( , ), one being used as HV-elec-trode, the other for readout (cf. the two BNC connectors at thebottom of the box). For timing tests, two FEE-1 cards [11], eachconsisting of four discrete channels of high-frequency ampli-fiers followed by fast leading edge discriminators, are used toreadout two identical detector modules placed one behind theother perpendicular to the beam direction.

Using beam of 1 A GeV, a time resolution pswas measured (Fig. 13, right plot); a similar value pshas been obtained with Al ions of 2 A GeV.

B. An Integrated Universal Beam Monitor and Start Detectorfor Heavy-Ion Experiments

After the positive results obtained from our first SD test(Section IV-A) we designed a new assembly, in order to meetall the requirements of a start system of the FOPI experiment.


Fig. 13. The pcDD set used in a �� beam of 1 A GeV (left). The timedifference spectrum measured between two identical detectors (right). The timeresolution is 22 ps.

Fig. 14. Board comprising the 20 mm� 20 mm pcDD and the complete elec-tronics including two ASIC chips. The DD is divided into nine separate seg-ments for timing measurement and beam monitoring (see text for details).

The size of the pcDD was as large as 20 mm 20 mm; it is only0.15 mm thick in order to reduce beam straggling and reactionsin the detector material. It has identical aluminum contacts onfront and back side, consisting of a central electrode of 5.5 mmdiameter ( , ) for the start signal,four surrounding sectors for beam-profile measurements orbeam-halo control, plus another segmented outermost ring (cf.Fig. 14). The latter serves merely as a potential ring to guar-antee the field homogeneity along the borders of the beam-haloelectrodes.

In order to minimize the capacitances the FEE is equippedwith two PADI-1 chips [14]; all components are mounted to-gether with the DD on a common printed circuit board in a denseconfiguration.

Each PADI-1 chip has three preamplifier-discriminator chan-nels followed by digital buffers, which are used for readout andprocessing of the signals of the five detector segments; the dig-itization is performed with external units.

This setup has been used in an experiment with a 1.9 A GeVbeam. The time resolution was measured in reference to

the standard FOPI start detector, i.e., a plastic scintillation de-tector, read out by two fast phototubes [13]; the resolution ofeach sensor can be determined independently. From the mea-sured total ps (Fig. 15), ps are attributed tothe diamond setup.

The rate capability of PADI-1 has been investigated in a beamtest experiment with carbon ions using a scDD of 3 mm 3 mmarea. The diamond sensor of 0.3 mm thickness was metallizedwith four circular pad electrodes on both sides, each of 1.2 mmdiameter and of capacitances and

Fig. 15. Time difference spectrum measured with the central pad of the DD ofFig. 14 in coincidence with a plastic scintillation detector. The DD plus FEEcontributes with � � �� ps to the total � � �� ps of the ToF spectrum.

, respectively. We observed a linear dependence of thedetected rate on the beam intensity up to .For details see [17], [18].

C. Start Detectors for Relativistic Light Ions and ProtonsFeaturing a Simple MOS Buffer

The model calculations as well as the experimental resultsdescribed so far showed clearly that good time resolutions forlight ions, and in particular protons, demand additional designimprovement: one has to consequently minimize all relevant ca-pacitances of the assemblies, to increase the preamplifier inputimpedance, and, above all, to use single-crystal diamond as de-tector material.

For a first beam test, the electrodes of two scDDs (ofmm mm and thickness mm) have been segmented

into four sectors, each of those directly bonded to the corre-sponding input of its amplifier, which was located in close prox-imity of the sensor. We have chosen the BFR998 double-gateMOS transistor which is often used in VHF/UHF amplifierssince it has a very good noise figure (typically 0.6 dB@200 MHzand 1 dB@800 MHz) and a . In addition, it hasan input capacitance –2.4 pF, an output capacitance

, a feedback capacitance and atransconductance of . The DC bias and thedetector signal are transmitted through a 50 coaxial cable toa modified version of the FEE-1 [11] which contains also thebiasing circuit. The simple MOS buffer (MB) circuit (Fig. 16)allows to easily adjusting the detector time constant (in this case

). The resulting capacitances persector were and .

Fig. 17 shows the time difference spectrum measured withtwo well-aligned opposite sectors of the two quadrant sensors,which were mounted in short distance between each other per-pendicular to a beam of 1.8 A GeV. The time resolutionobtained was ps.

However, this simple MOS buffer solution has two draw-backs: is not sufficiently low, and, since the gainis below 1.0, the noise of the next cell must be low too.


Fig. 16. Schematics of the MOS buffer.

Fig. 17. Time difference spectrum measured with relativistic �� ions of 1.8 AGeV using the MOS buffer plus the FEE-1 card and scDD sc3 (Table II).

Fig. 18. The new SD of HADES is a scDD of 4.7 mm � 4.7 mm size and athickness of 500 �m; it is segmented in eight pads.

D. The New scDD Proton Start Detector for HADES

In the design described in the following, the above disadvan-tages have been eliminated. For the new proton start detector forthe HADES spectrometer at GSI, the segmentation of the sensorelectrode has been driven even further: the scDD of 4.7 mm4.7 mm area and mm thickness is segmented in eightpads (Fig. 18) yielding and .The new FEE consists of a low capacitance broadband amplifier(LCB) for the first stage mounted close to the sensor [19].

Fig. 19. Schematics of the low capacitance broadband amplifier FEE.

Fig. 20. Mounting details of the LCB: at left is shown the PCB with 8 channelsused in the proton beam test and on the right one, a quadrant-SD (sc4, Table II)surrounded by four LCB amplifiers, as used in a �� beam test.

Fig. 19 shows the used FEE schematic. The low capacitancebroadband amplifier (LCB) consists of a 14 GHz wideband sil-icon planar transistor mounted in common-emitter configura-tion. It is mounted as close as possible to the corresponding padof the segmented electrode. This transistor has a low noise figure

dB as well as low collector-baseand emitter-base capacitances . The biasingresistor is divided in three elements to minimize the total par-asitic capacitance. The collector load resistor is located at theinput of the next cell.

The left-hand side photograph of Fig. 20 shows the FEE boardwith its eight LCB channels before the mounting and bonding ofthe DD shown in Fig. 18 into the central hole. The identical elec-tronics scheme was used to test a second SD assembly, whichwas consisting of a sensor with one electrode segmented in foursectors (quadrant detector sc4, Table II). Its board is shown onthe right side of Fig. 20 with the SD mounted in the right hole.The collectors of the LCB amplifiers are connected throughmicro strip lines to coaxial cables, which transmit the signals tothe next amplifier cells. The PCB material is Rogers 4003C, ahigh-frequency material (glass-reinforced hydrocarbon and ce-ramics) with a dielectric constant . In order tominimize the detector fringe capacitance, the ground plane hasa circular groove cut around the detector.

Two identical single-crystal diamond quadrant sensors (cs4,Table II) were tested with ions of 1.8 A GeV and an timeresolution ps (Fig. 21) was achieved. This can be com-pared to the 55 ps measured in the same beam test with thesimplified MOS-buffer FEE type of Section IV-C (Fig. 17). Theeight-sector SD delivered with protons of 3.5 GeV a resolution

ps, which is the best proton value, reached so far [19].


Fig. 21. Time resolution measured with �� ions of 1.8 A GeV between twosegments of two different scDD (sc4, Table II), read out with the low capacitancebroadband amplifiers (LCB) developed for the HADES spectrometer.

TABLE ITIME RESOLUTIONS OF VARIOUS ASSEMBLIES TESTED WITH VARIOUS BEAMS

AND BEAM ENERGIES

TABLE IIDETAILS OF THE TESTED DD

V. SUMMARY AND CONCLUSIONS

In this paper we describe a variety of detector systemsconsisting of diamond detectors and their front-end electronics,which we have designed and built as in-beam start detectors forexisting and future GSI experiments. They should be able tocope with all beams of relativistic ions including protons, thedetection of which is a particular challenge because of the verylow primary charge generated in the detector material.

In parallel, we have performed extensive model calculationsto estimate the dependence of the signal-to-noise ratio andthe time resolution on various parameters. The computationresults presented in detail in Section III, together with the mea-sured resolutions of scDDs and pcDDs, show that a minor de-crease of the total system capacitance and an increase of theamplifier input resistance alone are not sufficient for protondetection. has to be minimized by a) an extremely carefuldesign of the DD-FEE assembly, where especially the first stage

Fig. 22. Comparison of measured time resolutions presented in this paper (fullsymbols) and other literature values (open symbols) for fast charged particlesfrom protons to U; square symbols denote scDD, round symbols denote pcDD.The added external values for pc-4, pc-5, pc-6, pc-7, pc-8 and sc-5 are takenfrom the references [21]–[25], respectively.

of the FEE has to be positioned as close as possible to the DD,b) a minimization of fringing detector capacitances (for whichthe mounting PCB should have the ground layers at a few mmdistance) and c) by segmentation of the DD into single pads, notexceeding a few mm . When is minimized, the input resis-tance can be adjusted to obtain the optimum condition for

: . In addition, one should use only single-crystaldetectors, which collect almost 100% of the primary charge.

Table I summarizes the experimental results obtained with thediamond detectors described in Table II: the time resolutions

achieved with the presented DD-FEE assemblies in variousbeam tests with relativistic ions of different energies, and thedetection efficiencies (EFF, if measured).

The tested scDDs and pcDDs are labeled sci and pci with i asrunning number; the FEE types are given as they are denomi-nated in the text. Along with the DD-FEE assemblies describedin Section IV, we present the results of an early test performedwith a scDD assembly (sc1, Table II) equipped with a FEE con-sisting of a fast charge sensitive amplifier (TCSA) [20] as a firststage followed by a FEE-1 card.

The pc diamond SDs read out with FEE-1 or PADI-1 ASICsdeliver satisfying resolutions for relativistic heavy-ions in theorder of ps. The sc diamond SDs have demonstratedtime resolutions ps with , and Al ions.

The best value obtained for relativistic protons wasps. This was achieved by segmentation of the electrodes

and reduction of parasitic capacitances and amplifier input ca-pacitance. In addition, the amplifier input impedance, and due tothis the detector time constant, had to be increased. The presentresolution is not satisfying yet in the light of the design goaldefined in the introduction. Further improvements are conceiv-able; however, both on the side of electronics and on the mate-rial side (e.g., the novel CVD diamond material, grown on largeiridium wafers has a good perspective for future timing applica-tions [5]).

In order to put our results in a larger context, we presentthe obtained values in Fig. 22 together with all other DD re-sults found in the literature. Ions from proton to and both


sc- and pc-material were used. It is obvious that our resultshave widened the field considerably; to our knowledge they arepresently the state of the art in timing with diamond detectors.

ACKNOWLEDGMENT

The authors express their gratitude to G. May and M. Trägerfor indefatigable cooperation and technical support. Specialthanks from all the authors go to P. Moritz for the excellent de-sign of the DBA amplifiers, and in particular to C. Kozhuharov,C. J. Schmidt, and H. Bozdog for many fruitful discussionsregarding this manuscript.

REFERENCES

[1] E. Berdermann and M. Ciobanu, “CVD-diamond detectors for experi-ments with hadrons, nuclei and atoms,” in CVD Diamond for ElectronicDevices and Sensors, R. S. Sussmann, Ed. New York: Wiley, 2009,pp. 227–255.

[2] H. Pernegger, S. Roe, P. Weilhammer, V. Eremin, H. Frais-Kolbl,and E. Griesmayer et al., “Charge-carrier properties in syntheticsingle-crystal diamond measured with the transient-current tech-nique,” J. Appl. Phys., vol. 97, p. 073704, 2005.

[3] M. Pomorski, “Electronic properties of single crystal CVD diamondand its suitability for particle detection in hadron physics experi-ments” PhD dissertation, Wolfgang Von Goethe University, Frankfurt,Germany, 2008 [Online]. Available: http://www-norhdia.gsi.de/publi-cations/pomorski_thesis_final_LQ.pdf

[4] W. Adam, W. de Boer, E. Borchi, M. Bruzzi, C. Colledani, and P. D’Angelo et al., “Radiation hard diamond sensors for future tracking ap-plications,” Nucl. Instr. Meth. A, vol. 565, pp. 278–283, 2006.

[5] E. Berdermann, M. Pomorski, W. de Boer, M. Ciobanu, S. Dunst, andC. Grah et al., “Diamond detectors for hadron physics research,” Dia-mond Related Mater., vol. 19, pp. 358–367, 2010.

[6] P. Moritz, E. Berdermann, K. Blasche, H. Stelzer, and B. Voss, “Broad-band electronics for diamond detectors,” Diamond Related Mater., vol.10, pp. 1770–1777, 2001.

[7] H. Spieler, SLUO Lecture Series 1998 [Online]. Available:http://www-group.slac.stanford.edu/sluo/lectures/detector_lec-ture_files/detectorlectures_10.pdf

[8] H. Spieler, Radiation Detectors and Signal Processing, Lectureseries pres, Univ. of Heidelberg, Germany, 2005 [Online]. Avail-able: http://www-physics.lbl.gov/spieler/Heidelberg_Notes_2005/pdf/IV_Signal_Processing.pdf

[9] H. Spieler, “Fast timing methods for semiconductor detectors,” IEEETrans. Nucl. Sci., vol. 29, no. 3, pp. 1142–1158, Jun. 1988.

[10] A. Pullia and E. Gatti, “Optimal filters with constant-slope crossoverand finite width for pulse-timing measurements,” IEEE Trans. Nucl.Sci., vol. 49, no. 3, pp. 1170–1176, Jun. 2002.

[11] M. Ciobanu, A. Schüttauf, E. Cordier, N. Herrmann, K. D. Hilden-brand, and Y. J. Kim et al., “A front-end electronics card comprising ahigh gain/high bandwidth amplifier and a fast discriminator for time-of-flight measurements,” IEEE Trans. Nucl. Sci., vol. 54, no. 4, pt. 3, pp.1201–1206, Aug. 2007.

[12] The APLAC Simulator [Online]. Available: http://web.awrcorp.com/Usa/Products/APLAC

[13] A. Gobbi, J. P. Alard, G. Augustinski, Z. Basrak, N. Bastid, and I. M.Betayev et al., “A highly-segmented��-time-of-flight wall as forwarddetector of the ��-system for charged particles at the SIS/ESR accel-erator,” Nucl. Instr. Meth. A, vol. 324, pp. 156–176, 1993.

[14] M. Ciobanu, N. Herrmann, K. D. Hildenbrand, M. Kis, and A. Schüt-tauf, “PADI, a fast preamplifier—Discriminator for time-of-flight mea-surements,” in Proc. IEEE Nucl. Sci. Symp., Dresden, Germany, 2008,vol. N30-18, pp. 2018–2024.

[15] G. Agakishiev, C. Agodi, H. Alvarez-Pol, E. Atkin, E. Badura, andA. Balanda et al., “The high-acceptance dielectron spectrometerHADES,” Eur. Phys. J. A, vol. 41, pp. 243–277, 2009.

[16] W. C. Chew and J. A. Kong, “Effects of fringing fields on the capaci-tance of circular microstrip disk,” IEEE Trans. Microw. Theory Techn.,vol. MTT-28, no. 2, pp. 98–104, Feb. 1980.

[17] M. Rebisz, “Alternative methods for heavy-ion therapy dosimetry”Ph.D. dissertation, Ruprecht-Karls Universität Heidelberg, Germany,2007 [Online]. Available: https://www.gsi.de/documents/DOC-2008-Apr-5-1.pdf

[18] M. Rebisz-Pomorska, M. Ciobanu, M. Kis, M. Pomorski, and B. Voss,“Diamond detectors for the monitoring of carbon-ion therapy beams,”Nucl. Instr. Meth. A, vol. 620, pp. 534–539, 2010.

[19] J. Pietraszko, L. Fabbietti, W. Koenig, and M. Weber, “Diamond astiming detectors for minimum-ionizing particles: The HADES proton-beam monitor and START signal detectors for time of flight measure-ments,” Nucl. Instr. Meth. A, vol. 618, pp. 121–123, 2010.

[20] M. Ciobanu, N. Herrmann, K. D. Hildenbrand, T. I. Kang, M. Kis,and A. Schüttauf, “A charge sensitive amplifier for time and energymeasurements,” in Proc. IEEE Nucl. Sci. Symp., Dresden, Germany,2008, vol. N30-20, pp. 2028–2032.

[21] E. Berdermann, K. Blaske, P. Moritz, H. Stelzer, B. Voss, and F. Zey-touni, “First applications of a CVD-diamond detectors in heavy-ion ex-periments,” Nucl. Phys. B (Proc. Suppl.), vol. 78, pp. 533–539, 1999.

[22] M. Mikuz, V. Cindro, I. Dolenc, H. Kagan, G. Kramberger, andH. Frais-Kölbl et al., “The ATLAS beam conditions monitor,” inProc. IEEE Nucl. Sci. Symp., Honolulu, HI, 2007, vol. N31-7, pp.1914–1917.

[23] A. Lostroh, F. Schirru, B. S. Nara Singh, L. Scruton, M. Bentley, andS. P. Fox et al., “Polycrystalline diamond detectors for time of flightmeasurements of high energy ion beams,” presented at the IEEE Nucl.Sci. Symp., Knoxville, TN, 2010.

[24] A. Stolz, M. Behraven, M. Regmi, and B. Golding, “Heteroepitaxialdiamond detectors for heavy ion beam tracking,” Diamond RelatedMater., vol. 15, pp. 807–810, 2006.

[25] E. Berdermann, K. Blasche, P. Moritz, H. Stelzer, and B. Voss, “Theuse of CVD-diamond for heavy-ion detection,” Diamond RelatedMater., vol. 10, pp. 1770–1777, 2001.

In-Beam Diamond Start Detectors

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