Time Projection Chamber (TPC) Detectors for Nuclear Astrophysics Studies With Gamma Beams

TL;DR

This paper reviews the development and application of Time Projection Chamber (TPC) detectors with gamma beams at HIgS and ELI-NP facilities, aiming to improve nuclear astrophysics measurements like the 12C(a,g) reaction.

Contribution

It presents recent progress and future plans for optical and electronic readout TPC detectors in gamma-beam facilities for nuclear astrophysics research.

Findings

Successful implementation of optical readout TPC at HIgS.

Development of electronic readout TPC for ELI-NP.

Enhanced capabilities for measuring nuclear reactions relevant to astrophysics.

Abstract

Gamma-Beams at the HIgS facility in the USA and anticipated at the ELI-NP facility, now constructed in Romania, present unique new opportunities to advance research in nuclear astrophysics; not the least of which is resolving open questions in oxygen formation during stellar helium burning via a precise measurement of the 12C(a,g) reaction. Time projection chamber (TPC) detectors operating with low pressure gas (as an active target) are ideally suited for such studies. We review the progress of the current research program and plans for the future at the HI{\gamma}S facility with the optical readout TPC (O-TPC) and the development of an electronic readout TPC for the ELI-NP facility (ELITPC).

Full text

††journal: Journal of LaTeX Templates

1 Introduction

Gamma-Beams

Gamma-beams (2–20 MeV) proved to be enormously useful for low energy nuclear physics studies in the pioneering work at the High Intensity Gamma-ray Source (HI$\gamma$S) facility at the Triangle Nuclear Physics Laboratories (TUNL) located at Duke University in the USA [1]. Further improvement of the energy resolution (by a factor 5) and intensity (by a factor of 10) anticipated for the Extreme Light Infrastructure – Nuclear Physics (ELI-NP) facility under construction at Magurele near Bucharest in Romania [2], promises to allow some of the most crucial measurements in nuclear astrophysics. Specifically, the C/O ratio at the end of stellar helium burning has been emphasized as a problem of “paramount importance” in nuclear astrophysics [3]. To solve this problem we need to measure with high accuracy the cross section of the 12C($\alpha,\gamma$) reaction at low energies approaching center of mass energy of 1.0 MeV and resolve the nagging ambiguities [4] in the extrapolated values of the p-wave and d-wave cross sections at the Gamow window (300 keV), designated by SE1(300) and SE2(300), correspondingly. The high intensity and improved energy-resolution anticipated for the gamma-beam of the ELI-NP provides a unique opportunity for a high precision measurement of the 12C($\alpha,\gamma$) reaction at Ecm = 1.1 MeV by measuring the inverse 16O($\gamma,\alpha$) reaction with a gamma-beam of Eγ = 8.26 MeV. A detailed and complete angular distribution spanning the entire angular range of 0–180∘ appears possible with a three-week measurement [5]. Such a complete angular distribution spanning the entire angular range of 0–180∘ was demonstrated [6] to permit a separation of the E1 and E2 cross sections and the corresponding mixing phase-angle ($\phi_{12}$) with very high precision.

Optical TPC Detectors

Optical readout tme projection chamber (TPC) detectors were used in pioneering measurements with radioactive beams at the NSCL in MSU [7, 8] and with gamma-beams at the HI$\gamma$S facility [6, 9]. But these detectors use optical readout and suffer from low counting rates that does not permit the use of the full beam power (even at the HI$\gamma$S setup). In order to fully utilize the high intensity anticipated for the ELI-NP gamma beam an electronic readout (eTPC) concept has been developed [5]. A mini-TPC [10] prototype of the full detector planned to be used at the ELI-NP facility (ELITPC) was constructed at the University of Warsaw and delivered to the ELI-NP facility.

2 Measurements at the HI$\gamma$S With the O-TPC

Data on the 16O($\gamma,\alpha$) reaction

A large volume of data (approximately 4 TB) collected at the HI$\gamma$S facility are now being analyzed [11, 12]. The goal of this analysis is two fold: First, we plan to extract with high precision the E1 and E2 cross sections and the E1-E2 mixing phase angle ($\phi_{12}$) measured in the angular distribution of the 16O($\gamma,\alpha$) reaction with gamma-beams at Eγ = 9.08, 9.39, 9.58, 9.78 MeV with two different gas mixtures CO2(80%) + N2(20%) and N2O (80%) + N2(20%), at 100 Torr. Second, the development of the analyses routine will prepare us to analyze the data anticipated from measurements with the ELITPC. After completing the current HI$\gamma$S data analyses we plan to use the 200 hours of already approved beam time to measure at the lowest possible energy at the HI$\gamma$S facility (constrained by the count-rate) at Eγ = 8.80 MeV.

Measurement with N2O gas mixture

The use of N2O gas mixture which was developed at the Weizmann Institute [13] proved to be very beneficial since it removes the background from the 12C($\gamma,3\alpha$) reaction. But the poor energy resolution obtained with the N2O gas mixture (most likely due to e-N2O resonance) prevented the use of the anode grid signal to measure the total energy deposited and necessitated relying on the measured track length to separate reactions from the dissociation of 16O and 18O with $\Delta$Q = 935 keV. In Fig. 1 we show a typical event measured with the N2O gas mixture including the track recorded by the CCD camera [9] and the time projection PMT signal [9]. We are currently analyzing all available data to extract the measured angular distributions with the N2O gas mixture [11].

3 The ELITPC Detector

The proposed ELITPC detector

The ELITPC detector proposed by the charged-particle working group [5] has been reviewed by the ELI International Scientific Advisory Board (ISAB) and was approved for construction and installation at the ELI-NP. Briefly, it utilizes an electronic readout in the horizontal plane perpendicular to the drifting electrons, of three lines oriented at 120∘ to each other placed on a multi-layer PC board (commonly referred to as a u-v-w readout). The electron multiplication is achieved with three 35x20 cm2 Gas Eelectron Multipliers (GEMs). The ELITPC is proposed as one of the two main detectors for measurement of charged particles of relevance to nuclear astrophysics as discussed in [5].

The mini-TPC Prototype Detector

A smaller mini-TPC detector has been constructed at the University of Warsaw [10] in order to study and optimize the performance characteristics of the ELITPC. The homogeneity of the electric field was simulated using MAXWELL [14] to be better than 1 V/cm ($<$0.5%) and the results are shown in Fig. 2.

Test of the mini-TPC at the IFIN Tandem

The mini-TPC was tested with alpha-beams extracted from the IFIN tandem as well as with neutrons produced by the same alpha-beam with a Be target. In Fig. 3 we show an event of 16O dissociation by a neutron vividly displaying the reconstructed alpha-particle and 12C tracks.

4 Conclusion

In the analyses of the HI$\gamma$S data, 16O and 18O dissociation events have been identified and differentiated from background and angular distributions are being generated. The first measurement of 16O dissociation at the IFIN shown in Fig. 3 serves as a “proof of principle” of the design specs of the ELITPC which is now ready to move to the construction and installation phase at the ELI-NP.

5 Acknowledgements

The material presented in this paper is based upon work supported by the U.S. Department of Energy, Office of Science, Nuclear Physics, Award No. DE-FG02-94ER40870 and DE-FG02-91ER-40608. Scientific work is supported by the Polish Ministry of Science and Higher Education from the funds for years 2017–2018 dedicated to implementing the international co-funded project no. 3687/ELI-NP/2017/0 and by ELI-NP/IFIN-HH under the Collaborative R&D Project Agreement no. 88/25.10.2016. The ELI-NP team acknowledges the support from the Extreme Light Infrastructure Nuclear Physics (ELI-NP) Phase II, a project co-financed by the Romanian Government and the European Union through the European Regional Development Fund - the Competitiveness Operational Programme (1/07.07.2016, COP, ID 1334).

6 References