Fast Timing with Diamond Detectors

Lianne Scruton

Outline

• Why do we need fast timing?• The LYCCA array• What makes diamond an excellent timing detector?• Constructing and testing the diamond detector• The diamond detector with LYCCA• Comparing diamond with plastic scintillator• Future plans for LYCCA

• Quite simply: for identification

• A better timing resolution makes the time-of-flight measurements more precise.

• High rate measurements possible.

Fast Timing: Why?

Start Signal

Stop Signal

Target

• FAIR facility under construction at GSI

What are we using it for?

July 2013

• FAIR facility under construction at GSI

What are we using it for?

FRS

• The Super-FRS will allow for cleaner, more intense secondary beams.

• The HISPEC (High-resolution In-flight SPECtroscopy) campaign will be located at the end of the Super-FRS.

• HISPEC will focus on in-flight decays of exotic nuclei to study collective motion, position of neutron dripline and much more.

• Need something to track and identify these exotic nuclei.

What are we using it for?

• LYCCA: Lund-York-Cologne Calorimeter

• The design of LYCCA is based on CATE (CAlorimeter TElescope).

Introducing: The LYCCA Array

Si array for ΔECsI array for residual E

(Lozeva 2006)

• Simulations performed by M J Taylor showed that including Time-of-Flight (ToF) detectors improved identification

Simulations

(Taylor 2009)

Data from CATE

• Simulations performed by M J Taylor showed that including Time-of-Flight (ToF) detectors improved identification

Simulations

(Taylor 2009)

(FWHM)

• Simulations performed by M J Taylor showed that including Time-of-Flight (ToF) detectors improved identification

Simulations

(Lozeva 2006)

(FWHM)

The LYCCA ArraySecondary

TargetStart

DetectorStop

Detector

Si DSSSDs for ΔE and tracking

CsI scintillator for residual E

3.6 m

Si DSSSD for tracking

The LYCCA Array

• LYCCA energy detectors make up a modular wall that can be arranged into different configurations

• Maximum number of modules is 26 which covers an area of over 1000 cm2

The LYCCA Array

• Heavy charged particles pass through the semiconductor and interact with electrons in the material via the Coulomb interaction.

• Electrons are excited across the band gap into the conduction band creating electron-hole (e-h) pairs.

• Applying a bias across the semiconductor allows electrons and holes to travel to opposite contacts, inducing charge on contacts.

Semiconductor Detectors

e-

h+

e-

h+

e-

h+

+ 600 V

0 V

Contact

Contact

Semiconductor

• Charge builds up on contacts until charge carrier motion ceases.

Signal Generation

Tstart

Tstart

• Rise time of current pulse is unaffected by the interaction point.

Signal Generation

• Diamond has a number of properties that are advantageous for timing measurements:

1. Electrons and holes have high and similar mobilities

2. High optical phonon energies lead to high saturation velocity

3. Wide band gap – low dark current and noise

4. Diamond has a low dielectric constant – small capacitance

• Resultant current pulse is short with a large amplitude and a rise time with a steep gradient.

Why Diamond?

• Large size of diamond start detector meant that polycrystalline diamond must be used which contains grain boundaries.

• Other impurities (B, N etc.) can also act as traps.

Polycrystalline Diamond

(Hammersberg 2001)

• For the best timing, we need to limit the number of impurities and grain boundaries.

Polarisation Fields

• Trapped charge carriers can no longer contribute to the signal current.

• Polarisation field set up by trapped holes and electrons

• This field opposes the electric field, reducing velocity of charge carriers.

Constructing the Diamond Detector

• Diamond detector consists of a 300 μm-thick diamond wafer sandwiched between two metallic contacts (Pt/Au, Au or Al).

• Top contacts are divided into four 18 x 4.5 mm2 strips to reduce the capacitance associated with the detector.

• Contacts with different pad sizes were used to test the effects of different capacitance on pulse.

• 50-MeV He4 beam scattered from Pb target.

B’ham Optimisation Test

14.6 pF

8.11 pF1.95 pF

14.6 pF

B’ham Optimisation Test

• Diamond start detector placed ~5cm downstream of target.

• Plastic scintillator used as second ToF option with start and stop scintillators upstream and downstream of target.

LYCCA and the Diamond detector

Commissioning Experiment – Sep 2010

DSSDDSSDDSSD Wall

CsI

Plastic ToF

Diamond ToF

3.6 m

4.3 m

Identifying the Fragments

Mass res = 0.55 ± 0.02 u (FWHM)

Time res = 50.8 ± 2.4 ps (FWHM)

• Beam velocity, β, and energy used to calculate fragments on an event-by-event basis.

Mass Measurements

Diamond ToF

Mass res = 1.27 u

Plastic ToF

• Time resolution of diamond detector = 193.0 ± 25.6 ps• Resolution of 104 ps obtained at Texas A & M test…

What Went Wrong?

• Cables used between detector and preamp were ~2 m long. Added unwanted capacitance to detector circuit.

• Gradient of signal is shallow and noisy.

What Went Wrong?

• Noise described as: (Ciobanu 2011)

• Require electronic development to overcome this issue.

• Decision made to use plastic scintillator for LYCCA ToF.

• 1 PMT has poor timing resolution, but 56 measurements of same event improves resolution .

The New Plastic Scintillators

• Fast timing detectors require current signals that are large and short with a fast rise time.

• Fast charge carrier mobilities, high saturation velocity and low dielectric constant make diamond ideal for fast timing.

• Diamond detector and plastic scintillator timing options were compared in first LYCCA commissioning experiment.

• Clear isotopic resolution obtained using plastic scintillator timing , but poor resolution for diamond ToF.

• Poor performance of diamond attributed to the necessarily long cable lengths used between detector and preamplifier.

• New plastic stop scintillator detector under development here at York for use with LYCCA at FAIR.

Summary

References• R Lozeva et al. Nuclear Instruments and Methods A, 562, pg 298-305 (2006)

• M J Taylor et al. Nuclear Instruments and Methods A, 606, pg 589-597 (2009)

• J Hammersberg et al. Diamond and Related Material, 10, pg 574-579 (2001)

• M Ciobanu et al. IEEE Transactions on Nuclear Science, 58, pg 2073-2083 (2011)

Thank you for Listening