Overview

GRIT stands for “Granularity, Resolution, Identification, Transparency”. It is a new generation silicon array for low energy nuclear physics built conjointly between IPN Orsay, INFN-Padova-LNL, the University of Surrey, the LPC Caen, BARC MUMBAI and the University of Santiago de Compostela.

GRIT offers the unique possibility to investigate nuclear structure and nuclear astrophysics with high efficiency and high resolution by coupling with state-of-the-art gamma array like AGATA, PARIS, EXOGAM2, MINIBALL…

 

Physics

GRIT aims at measuring direct reactions of nuclear strucutre and/or nuclear astrophysics interest.  Direct reactions are a powerful tool in nuclear physics. The relative simplicity of the reaction mechanism involving few degrees of freedom, allow precise theoretical calculations to be made and nuclear structure information to be extracted from experimental data.

Performed in inverse kinematics, where a radioactive beam impinges on a light target (H, He,…), they require the measurement and the identification of the recoiling light particle with high accuracy. Silicon based detectors are particularly well-suited as they combine high granularity with good energy resolution. Combined with  gamma arrays, they can benefit from their excellent energy resolution (few keV) and achieve excellent performances. This is why GRIT is designed to be coupled with state-of-the-art gamma arrays like AGATA, PARIS, EXOGAM2 or MINIBALL.

Nuclear chart with example of direct reactions to study nuclear structure and nuclear astrophysics

Several Letters of Intent have been written to perform experiments at HIE-ISOLDE, SPES, FAIR and Spiral2

Design

The GRIT array is composed of :

  • 2 rings of trapezoidal telescopess in the forward and backward direction
  • 1 ring of square telescopes at 90 degrees
  • 2 annular detectors in the most forward and backward angles

Each telescope consist of :

  • one 500 um thick first layer of nTD (neutron transmutation doped) type Double Sided Stripped Silicon detectors (DSSSD) with 128 strips on each side
  • one or two (in the forward hemisphere) 1.5 mm thick DSSSD with larger pitch (32 strips on each side).

The thin trapezoidal and square prototypes have been developed and fully tested on a test bench. The energy resolution is of about 50 keV all strips summed.

A thick (1.5mm) square detector has also been developed and tested with an alpha source. The obtained energy resolution is of about 70 keV (FWHM).

Electronics

Given the number of strips to be read (~2000) in the GRIT project, highly integrated electronics is required.

Two ASICs will be used in the project: the iPACI preamplifiers, which give charge and current information and the PLAS analog memory, that sample the signals before sending them to the back-end electronics.

iPACI

  • The iPACI (integrated Charge and Current preamplifier) ASIC is being developed at IPN for the DSSSD readout. It will be fed with the DSSSD outputs and will embed the required hardware to extract the charge, current and time information.

The first version of iPACI, is a 9-channel read-out chip with independent charge and current outputs. The architecture of the chip is shown in Fig.7. It is composed of nine identical channels, each comprising a charge-and-current preamp, and two output buffers. The performance measured on a testbench (without detector and/or alpha source) is summarized in Tab. 1.

Table 1 : Measured performances of iPACI v1

 

Charge output Current output
Energy max 50 MeV current gain 7 kΩ
Charge gain 32 mV/MeV current swing 1.5V single ended
Equivalent noise Charge 15 keV current BW [4MHz .. 120MHz]
Charge non-linearity <1%
Charge output recovery 100 µs
Detector’s input capacitance Compatible with [10pF .. 40pF] range
Current consumption 12mA (40mW) / Channel
  • A second version of the ASIC, called iPACI v2, has been designed. It is a 16-Channel particle detector read-out chip, which integrates several common functions, i.e. slow shaper, Time-to-Amplitude Converter (TAC), serializer, fast shaper. The ASIC embeds slow control circuitry, enabling real-time tuning. Fig. 8 shows the chip’s architecture.

Keeping the same philosophy as for iPACI v1, the charge and current signals can be selectively sent out. Two ranges in energy are implemented in order to read out thin detector signals for light and heavier particles.

Other features like fast and slow shaper have been implemented in order to be able to read a possible second layer of Silicon. In this case, pulse shape analysis would not be needed as the ΔE-E technique already gives the particle identification. Tab. 2 gives a summary of the simulated performance of iPACI v2.

 

PLAS

The University of Valencia (R. Aliaga, V. Herrerro-Bosch) is developing an analog memory ASIC called PLAS (ALI15), for PipeLined Asymmetric SCA for the Silicon array. A novel 32-input analog memory, with self-triggering channels for the sampling of detector’s pulses and their transmission at slower pace. The pipelined asymmetric SCA reduces the number of SCA cells needed by a factor of 7 with respect to single full SCA, thus reducing the size of the chip and power consumption.

 

Sampling rate 200 MSa/s
Resolution 12 bits ENOB
Nb of entrance channels 32
Post-trigger slots 8
Cells depth 32+192
Trigger internal
Time stamping [10pF .. 40pF]

Table 3: Simulated characteristics of PLAS.

 

Captured signals are timestamped and transmitted serially to the back-end by means of single analog output, reducing considerably the number of feedthroughs. They are digitzed remotely and processed by FPGA (FASTER back-end) that controls read out process at 50 MHz. The 100 MHz sampling clock is handled by the back-end electronics. Both clock edges are used for the sampling to reach 200 MSa/s.

 

MUGAST

MUGAST is an intermediate step in the GRIT project. In order to benefit from the AGATA-VAMOS campaign at GANIL and from the development of the new Spiral1 beams, the new detectors developed for GRIT have been associated to the MUST2 array. The MUGAST configuration consists of 5 trapezoidal detectors in the backward direction, 2 square detectors at 90 deg. and 4 MUST2 telescope at forward angles, offering a wide angular coverage. All the detectors are read by the MUST2 electronics. With this arrangement, AGATA is at 18 cm from the target.

The MUGAST campaign will run at GANIL beginning of 2019.The reaction chamber can accomodate specific targets like cryogenic ones. In particular, an 3He and 4He cryogenic target will be available during the campaign in 2019.