Frascati Beam-Test Facility beam divergence characterization with Silicon micro-strip detectors

G. Ballerini, A. Berra, S. Costa, E. Marchi, G. Pozzoli, M. Prest, L. Scali, M. SoldaniUniversità dell’Insubria, Como

B. Buonomo, C. Di Giulio, L. FoggettaINFN Laboratori Nazionali di Frascati

P. ValenteINFN Sezione di Roma

E. VallazzaINFN Sezione di Trieste

AbstractThe described measurements were carried on in the framework of the undergraduate students course of “Laboratory IV” (dedicated to nuclear and particle physics laboratory experiences) of the Insubria University (Como, Italy) in close collaboration with the Frascati Beam-Test Facility (BTF), with two main objectives:

- Allow a selected team of brilliant undergraduate students having a real hands-on experience on a setup typical of high-energy experimental physics, with complex state of the art detectors;

- Test a tracking device during the every day operation of the facility, with no specific beam optimization, in order to have a better definition of the needed improvements on both the beam parameters and the diagnostic tools of the facility, in view of the planned upgrades in the framework of the AIDA-2020 project, Work Package 15.

Introduction: BTF description and operationThe BTF has been designed as a part of the DA NE complex: it is composed of aΦ transfer line driven by a dipole magnet allowing the diversion of electrons or positrons, usually injected into to the damping ring, from the high intensity LINAC towards a dedicated experimental hall.

The facility can provide runtime tuneable electron and positron beams in a defined range of different parameters, depending on the choice of one of the following two main operation modes: “single particle” regime, in which the beam is attenuated by means of a step Copper target (1.7, 2 or 2.3 X0), producing a secondary beam with a continuous full-span energy (from LINAC energy down to few MeV) and intensity, down to a regime in which the particle multiplicity per bunch follows a Poisson distribution; high-intensity beam extraction, when the full LINAC beam is steered in the BTF hall, with some intensity tuning by means of collimating Tungsten slits.

The layout of the transfer lines at the end of the LINAC is shown in Fig. 1.1.

Figure 1.1: Layout of the transfer lines at the end of the LINAC.

During DANE collider operations, one of the 50 bunches accelerated by the LINAC in one second is driven to the 6° spectrometer line (by the DHPTS01 pulsed 45° bending), conventionally the last of a 1 second long sequence.

Out of injection, both in electron and positron mode, all the remaining 49 bunches are available and are driven to the BTF line at 3° by the DHPTB101 dipole.

During injections, after that the required number n of electron or positron packets have been driven on the straight line towards the damping ring (both DHPTB101 and DHPTS01 off), and subtracted the two buckets reserved for the energy measurement (since the extraction from the damping ring to the main rings runs at 2 Hz, also the number of pulses to the spectrometer in 1 second is doubled), the remaining 48-2n bunches can be driven to the BTF line (DHPTS01 off and DHPTB101 on).

The BTF beam optics can be optimized by the elements downstream the selecting dipole DHSTB01: two FODO quadrupole doublets (QUATB01-02 and QUATB03-04, as shown in Fig. 1.2) are placed along the ~10 m long line, one in the LINAC tunnel, and one inside the BTF experimental hall, where the beam pipe enters at 45° with respect to the room walls. In order to drive the beam along the main axis of the room, another 45° static dipole (DHSTB02) is used, identical to the one used for energy selection, after the last quadrupole doublet inside the BTF hall.

This last dipole (DHSTB02) has a two-fold exit, so that the beam can also be available on a straight section of the vacuum pipe (dipole off), essentially for diagnostics purposes, due to the limited space in front of this beam exit.

Figure 1.2: General layout of the BTF line and experimental hall.

In single particle mode, i.e. from few up to few hundreds particles/bunch, standard particle detectors are integrated in the BTF diagnostics and routinely used, in particular Silicon pixel imaging detectors (of the Medipix family) and calorimeters (usually a 11×11 cm2 cross section, 21 X0 deep lead glass block, from the former OPAL experiment electromagnetic calorimeter).

Using the average value of the calorimeter signal for a single electron (in ADC counts), of a given energy Esel fixed by the setting of the DHSTB01 dipole, we are able to count the number of particles by dividing the total deposited by the single electron value: n=ADCtot/ADC1 on a bunch by bunch basis, as shown in the screen-shot of the BTF online monitoring system in Fig. 1.3.

Figure 1.3: BTF online monitoring window, showing the calorimeter energy spectrum (left) and the measured number of electrons for each pulse (right), obtained by dividing the total ADC counts by the value of the first electron peak ADC1 (see text). In this case,

the selected energy by DHSTB01 was 450 MeV.

AGILE-type silicon micro-strip trackerIn order to characterize the beam divergence, a device capable of recording hits in both the transverse coordinates (we indicate with x the horizontal coordinate and with y the vertical one) in two different points along the beam trajectory (z coordinate) is needed.

In the framework of the collaboration between the BTF scientific staff and the University of Insubria silicon detectors lab (INSULAB), we have realized a dedicated run, with the participation of the students of the “Laboratorio IV” course, dedicated to nuclear and particle physics laboratory experiences.

The detectors are standard AGILE silicon single-sided micro-strip, manufactured by Hamamatsu on a high resistivity (>4k cm) substrate and readout by means of the self-triggering, low power TA1 ASIC from IDEAS.

The detector is 410 m thick Silicon with sensitive area 9.5×9.5 cm2, with strips every 121 m and with the so-called floating-strip readout scheme, so that the readout pitch is 242 m. Each of the four detector (two couples with x and y oriented strips) has 384 channels.

The Silicon tile is fully depleted with a bias voltage of 30 V, and a average leakage current of 1.5 nA/strip.

The readout electronics is based on custom VME-bus modules, designed by INFN-Trieste, with a readout time of 100 ms perfectly adequate for the BTF maximum repetition rate of 50 Hz.

The DAQ system is based on VME-bus standard, controlled by a SBS-Bit3 controller, optically linked to a Linux computer, running a custom software, written in C with Tcl/Tk graphical interface.

The setup for the measurements at BTF was the one shown in Fig. 1.4: one couple of Silicon micro-strips tiles was placed at about 25 cm from the exit window of the BTF beam line, while the second couple was placed at 23 cm distance, further downstream.

Figure 1.4: Layout of the two x-y Silicon micro-strip detectors installed at the exit of the BTF beam-line.

Experimental resultsIn order to determine the parameters of straight tracks crossing the two pairs of x-y detectors, we have selected events with single hits in each of the four Silicon micro-strip planes: x1 and y1 on the upstream pair of tiles and x2 and y2 on the downstream one. The resulting hit maps are shown in Fig. 1.5, for a typical “single particle” run at 450 MeV beam energy. The effect of the Tungsten collimators of the BTF beam-line is clearly visible.

Figure 1.5: Hit map on the two x-y Silicon micro-strip detectors y1 vs. x1 (upstream) and y2 vs. x2 (downstream) for an electron beam at 450 MeV.

In order to estimate the straight line parameters from the couple of hits, (x1,y1) and (x2,y2), on the four Silicon planes, the nominal distance among the two planes pairs of 23 cm has been used, as shown in the scheme of Fig. 1.6.

Figure 1.6: In order to estimate the straight line parameters from the (x1,y1) and (x2,y2) hits, the nominal distance of 23 between the two detector planes has been used.

In Fig. 1.7 the reconstructed track angles x=(x2-x1)/d and y=(y2-y1)/d for a electron run at 450 MeV are shown. The distribution shows a Gaussian core, with =3.7 and 3.6 mrad in the horizontal and vertical angle, respectviely, with wider tails, as expected from the multiple Coulomb scattering.

Figure 1.7: Reconstructed track angles x=(x2-x1)/d and y=(y2-y1)/d for a electron run at

450 MeV. A Gaussian fit is also shown.

Taking into account the approximate formula for the RMS multiple scattering angle, as a function of the particle velocity and the thickness of the crossed material:

0=13.6/cp z (X/X0)½ [1+0.038 ln(X/X0)]

where p is the particle momentum in MeV/c, c the velocity, z the charge of the particle, X the thickness and X0 the radiation length of the target. Considering that a single detector plane is 410 m thick and is made of Silicon (X0=9.36 cm), the RMS angle introduced by the four Silicon planes can be estimated as 0≈0.432/p (MeV).

The effect of a tilt angle between the detectors and the nominal beam trajectory is evident when plotting the horizontal angle x as a function of the impact horizontal coordinate x1, as shown in Fig. 1.8, since the detectors were not aligned precisely but the beam was centered on the detector by adjusting the setting of the DHSTB02 dipole: an angular tilt of 36 mrad, corresponding to about 2, can be measured. A band of hits showing a correlation between the measured angle x and the impact point x1 can also be clearly seen, due to not focussed electrons.

Figure 1.8: Reconstructed horizontal track angle x=(x2-x1)/d as a function of the horizontal coordinate x1 on the first detector plane, for a electron run at 450 MeV. A

portion of the beam is focussed approximately at the center of the detector (x≈5 cm).

In order to make the tails of the beam more evident, in Fig. 1.9 the transverse coordinates on the first detector pair (x1,y1) is shown in logarithmic scale.

Figure 1.9: Reconstructed coordinates on the first detector pair (x1,y1), for a electron run at 450 MeV (logarithmic scale).

A similar analysis has been performed for an electron run at 197 MeV energy: the resulting beam spot is larger, especially in the horizontal coordinate, due to the dispersive effect of the DHSTB02 dipole, as shown in Fig. 1.10.

Figure 1.10: Reconstructed coordinates on the first detector pair (x1,y1), for a electron run at 197 MeV (logarithmic scale).

In Fig. 1.11 the horizontal and vertical angle distributions are shown, together with a Gaussian fit, yielding 7.4 and 7.6 mrad sigma for x and y respectively, again fully compatible with the expectation of the multiple scattering effect on the 1.64 mm of Silicon (1.75% of X0)

Figure 1.11: Reconstructed track angles x=(x2-x1)/d and y=(y2-y1)/d for a electron run at 197 MeV. A Gaussian fit is also shown.

ConclusionsThe described test with tracking detectors was useful in order to characterize the angular spread of the BTF beam in “normal” running conditions (no specific optimization of the beam spot, pretty standard settings of the optics and of the collimators) and to better define the requirements for the diagnostics of the BTF for specific low divergence applications: with 242 m pitch Silicon micro-strip detectors of 410 m thickness the dominant contribution is the Coulomb multiple scattering on the detector material itself. This indicates that an optimized tracking telescope for a beam in the range of 1 mrad or below divergence has to be pushed both in terms of the point resolution and thickness.