HPGe detectors at n_TOF

R.Vlastou, M.Kokkoris, Τh.Stamatopoulos, V.Michalopoulou,

National Technical University of Athens, Department of Physics, Greece

L.Tassan-Got, A.Tsinganis, M.Diakaki, E.Chiaveri, M.Barbagallo

CERN, Geneva, Switzerland

T.Glodariu†, A.Negret, A.Oprea, C.Petrone

Horia Hulubei National Institute of Physics & Nuclear Engineering, Romania

N.Patronis, Z.Eleme

University of Ioannina, Physics Department, Greece

R.Dressler, D.Schumann

Paul Scherrer Institute, Lab. of Radiochemistry and Environmental Chemistry

D.Bosnar

Department of Physics, Faculty of Science, University of Zagreb, Croatia

n_ToF Collaboration Meeting, November 27 – 29, 2018, Granada, Spain

Lack of experimental data on reactions important for basic nuclear physics, testing nuclear model parameters, applications, fast neutron reactors, etc. :

• inelastic scattering (n,n’) • (n,xn) reactions – high threshold reactions • Spectroscopy of fission fragments

Methods: • Direct neutron detection - difficulties to distinguish neutrons emitted from

elastic, inelastic , (n,2n), fission etc. processes • Activation technique is limited to residual nuclei with reasonable half-lives • Prompt gamma-ray spectroscopy – gammas emitted by the excited residual

nucleus

γ-spectroscopy measurements at n_TOF will open a vast field of research at n_TOF in the MeV energy region

Motivation

Problems with measurements implementing HPGe detectors in neutron spallation facilities

due to the saturation of the preamplifier caused by the γ-flash (producing huge energy deposition) and the long recovery time (to evacuate the charges

after the saturation) - the detector is blind for a few μs up to several ms

• In commercial detectors, with RC feedback circuit to restore the preamp output to baseline, if the energy-rate product deposited in the detector exceeds 120,000-180,000MeV/sec, the preamplifier saturates. By reducing the value of the feedback resistor the max energy-rate product can be increased but at the cost of increased noise and resolution degradation.

• With Transistor Reset Preamplifier (TRP), the feedback capacitor is discharged by means of a transistor switch connected to the FET gate. The energy gain is smaller (50mV/MeV instead of 100mV/MeV in RC preamps). Thus the preamp is active for much higher energy-rates and the saturation is avoided.

Problems at n_TOF

.

• 1st Test in September 2012 in EAR-1 • 2nd test in 2014 during the commissioning of EAR-2 • 3rd test in September 2015 in EAR-1 • 4th test in June 2017 in EAR-1

Using HPGe detectors from ISOLDE, PSI, Bucharest, Zagreb, conventional (with RC feedback circuit), with

commercial and homemade TRP (Transistor Rest Preamplifier), planar, coaxial, of 30-50% efficiency, at

different distances from the target holder, with low and high beam flux, with thick, thin, no target, etc.

History of HPGe tests at n_TOF

• Most of the pulses were saturated from

100μs up to 2ms in the worst case!

• The “γ-flash” is everywhere in the

experimental hall and not just close to

the neutron beam

• Planar with commercial TRP seems to

have a nice behaviour after the γ-flash-

Just a few saturated signals

• γ-ray pulses induced by energetic

neutrons could be clearly seen with

the planar detector

• BUT with very low efficiency for

energetic gamma-rays

History of HPGe tests at n_TOF

New pulse shape analysis routines have been developed, in Athens and Bucharest, to analyse the data from the planar detector, with signal recognition based on the first derivative of the pulses. Benchmarking of

this routine has been performed using calibration runs with 137Cs and 60Co sources

History of HPGe tests at n_TOF

Independent analysis by both Thanos Stamatopoulos and Andrea Oprea

Conclusions

Thus, it seems that even the commercial TRP is not enough to

face the saturation problems due to the gamma-flash at n_TOF.

Solution

In order to face the stringent operating conditions of n_TOF facility,

we proposed to Mirion technologies (CANBERRA) an R&D project

to develop a preamplifier allowing the discharge of the Ge crystal

during the gamma-flash burst, which saturates standard electronics.

The spectrum recording starts after this blind period.

Simulations have been performed by Mirion.

Finally, “Laurent’s switch” was adopted by Mirion : Isolate the

preamplifier from the detector during the γ-flash and drain the charges to

ground for a period that can be determined externally through the GATE.

New HPGe R&D

The new prototype HPGe was delivered in October 2018

• p-type HPGe

• Relative efficiency 26%

• Integrated switch circuit in preamplifier to avoid saturation from γ-flash

• Preamp conversion gain : Low-gain (10mV/MeV)

Tests of new HPGe at n_TOF

Tests by: Andrea Tsinganis, Veatriki Michalopoulou, Laurent Tassan-Got, Cristina Petrone and Zina Eleme

• Switch operation – screen shots from oscilloscope

Structure after the opening and closing of the gate (preamp’s response - can be optimized

through the circuit potensiometers)

Optimization of the structure after the gate

~ 200 ns after the beginning of the gate we see oscillations that cannot

be optimized - electronics

Tests in the Elab

The switch is off -> the preamplifier is plugged in the detector

The switch on -> The detector is grounded

• Switch response with source

Tests in the Elab

Beam tests @ EAR-1

• Tests performed with capture collimator, in beam with sources, without target, with thin Au and thick Si targets

• HPGe placed behind the TAC, PTB, PRT, 2-3m air (lots of scattered neutrons) at ~10 cm from the beam center

• PS trigger signal was send to the Old Control Room to generate the GATE through a pulse generator (~ 500-3500ns after the arrival of the γ- flash)

Tests of new HPGe at n_TOF

No Gate applied ---> Saturation for both dedicated and

parasitic ~ 6 ms

Gate ~ 200 ns after the γ-flash -> Limit for saturation of the electronics

Saturation is solved!

Beam tests @ EAR-1

Beam tests @ EAR-1

~ 45 MeV

Gate 500ns after the gamma-flash

However, we have the problem of the “Background” : its amplitude gets lower as the gate after the gamma flash gets higher (from 500ns to 3500ns), as we go far from the neutron beam (from 10 to 20 cm). Gets much higher with the presence of a thick target. Also scales with dedicated

and parasitic pulses Ιt's not coming from the gating of the switch but from the physics in the detector : scattered

neutrons and photons from the materials put into the beam (TAC, PTB, PRT, 2-3m air). We have to optimize the position and exp. conditions of the detector in the experimental area.

First impression from these tests is positive

First attempts to analyze the data

Special attention to the resolution in the analysis routine We need good resolution for gamma-ray spectroscopy

The most crucial point is the extraction of the signals in the first 10 µs We have to optimize the analysis routine

Spectrum of 88Y source : Analysis by Cristina Petrone Spectrum of 137Cs and 60Co sources : by Veatriki Michalopoulou

Analysis by Cristina Petrone

Treat the shape of the signals after the gate opens – subtract the gate response (blue) - recorded signal (red) and subtracted signal (green)

First attempts to analyze the data

• Fit the background in the first 30 µs with a function A0*(1-exp(-t/T1))*exp(-t/T2), where A0, T1, and T2 are parameters (red line)

• Subtract the fit from the original signal • The difference (green line) preserves all the

gamma signals • Easier signal recognition?

Another idea is the hardware cancellation of the “background”, before the preamp. That is in the same spirit as what we have done for the switch. The main problem is that the amplitude of the structure is pulse-dependent, so that we would have to inject a current (through a small capacitor), with a given known shape and an amplitude which is pulse dependent. This would allow to work with higher gain preamps and digitizers without saturating the frame and thereby improving the resolution.

Analysis by Laurent Tassan-Got

• Develop the right pulse-shape analysis routine - attention to resolution – treatment of the “background” after the gate

• Simulations to investigate the physics behind the “background” • Simulations to define the optimum position of the detectors in EAR1

(now it was placed behind the TAC) and the effect of target thickness • Simulations to define the geometry and functioning of the array • Simulations for the effect of possible shielding around the Ge crystal

to reduce the background • Decide the optimum cooling system (electric-hybrid cryostat) • Design and construct the support for the future array

Future work for the next 2 years

After the positive results of these tests If we want to gradually build a good array of 5-8 Germanium detectors within the next several years, we have to start a reliable and thorough

study of this project.

Thank you for your attention

Work in 2020 commissioning

• Further tests in EAR1 under optimal conditions, according to the simulations

• Try the hardware cancellation of the “background” • Try the detector with two different preamp gains • The design of the new Pb target at n_TOF will be appropriate to

optimize the experimental conditions (gamma-flash, gamma-ray background, neutron beam intensity) in EAR2 - Tests in EAR2

• Order 1-2 new detectors for the array • The first reactions that will be studied with this array are the inelastic

and (n,2n) reactions on 56Fe involving γ-ray transitions close to 1 MeV or 48Ti and 28Si, which are good benchmarks for the inelastic scattering.

Absolute efficiency at ~ 10 cm with 152Eu source

Energy calibration

Calibration of prototype detector

A large energy deposition in a short time (several GeV/100ns) would generate a drop in the crystal polarity of several volts. This is negligible as the crystal will be able to operate well above its depletion voltage

Efficiency simulation: ► A 25% relative efficiency germanium crystal is required ► Several geometries compliant with this request are considered ► 54mm x 54mm crystal size provides a good compromise over all the energy range Drift speed simulation: ► Crystal can be over depleted ► 2000-3000 V/cm intended ► Typical drift times: − 2cm maximum path in Ge − 200 ns typical drift times

Mirion simulations