Neutral-current neutrino cross section and expected supernova signals for 40Ar from a three-fold increase in the magnetic dipole strength

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Neutral-current neutrino cross section and expected

supernova signals for 40Ar from a three-fold increase in

the magnetic dipole strength

W. Tornowa,b, A. P. Tonchevc,a, S.W. Fincha,b, Krishichayana,b, X. B.

Wangd, A. C. Hayese, H. G. D. Yeomanse,f, D. A. Newmarke,g

aDepartment of Physics Duke University Durham NC 27708-0308 USA

bTriangle Universities Nuclear Laboratory Durham NC 27708-0308 USA

cNuclear and Chemical Sciences Division Lawrence Livermore National Laboratory

Livermore CA 94550 USA

dSchool of Science Huzhou University Huzhou 313000 China

eLos Alamos National Laboratory Los Alamos New Mexico 87545 USA

fKubrick Group London SE1 0BE United Kingdom

gDepartment of Physics Massachusetts Institute of Technology Cambridge Massachusetts

02139 USA

Abstract

In view of the great interest in liquid argon neutrino detectors, the 40Ar(γ, γ0)40Ar*

reaction was revisited to guide a calculation of the neutral current neutrino

cross section at supernova energies. Using the nuclear resonance ﬂuorescence

technique with a monoenergetic, 99% linearly polarized photon beam, we

report a three-fold increase in magnetic dipole strength at around 10 MeV in

40Ar. Based on shell-model calculations, and using the experimentally iden-

tiﬁed transitions, the neutral current neutrino cross sections for low-energy

reactions on 40Ar are calculated.

Keywords:

1. Introduction

Many current and planned neutrino facilities are based on liquid-argon

(LAr) detector designs, for example DUNE [1], the Deep Underground Neu-

trino Experiment, at the Sanford Underground Research Facility (SURF) in

South Dakota [2]. While the focus of these neutrino platforms is state-of-the-

art neutrino oscillation studies, most LAr projects are also poised to detect

Preprint submitted to Physics Letters B October 27, 2022

arXiv:2210.14316v1 [nucl-ex] 25 Oct 2022

neutrinos from core-collapse supernovae (SN). Although the charged-current

(CC) signal for SN neutrinos in LAr detectors has been studied [3] via the

reactions

νe+40 Ar →40 K∗+e−

and

¯νe+40 Ar →40 Cl∗+e+,

information on a neutral-current (NC) signal in the energy region of interest

is very limited. In this Letter we concentrate on the nuclear excitation of

40Ar nuclei in LAr by the NC neutrino interaction

ν+40 Ar →ν0+40 Ar∗,

and the corresponding, de-excitation γ-ray signals, which could shed light on

the physics of core-collapse SN neutrino bursts. The measured M1 and E1

strength was used to estimate the CC and NC cross sections for the neutrino

induced reactions on 40Ar.

Li et al. [4] reported the ﬁrst observation of a spin-ﬂip magnetic dipole

(M1) transition in 40Ar at 9.757 MeV with an assigned strength of B(M1↑

)=0.148(59)µ2

N. Shell-model calculations [4] suggested this state was one

fragment of the spin-ﬂip M1 strength in 40Ar. Recently, Gayer et al. [5],

studied dipole and quadrupole excited states of 40Ar between 4.2 and 7.7

MeV. In LAr detectors the de-excitation γrays are detected via electron-

positron pair production, favoring γrays of energies above the energy range

studied in Ref. [5]. The large uncertainty associated with the B(M1↑) value

in [4] and the possibility of locating additional M1 strength around 10 MeV

motivated us to revisit the 40Ar(γ, γ0)40Ar reaction and calculate the NC

cross section, which is closely related to B(M1↑) at very low energies [6].

2. Experimental measurement of magnetic dipole strength

The Nuclear Resonance Fluorescence (NRF) technique [7, 8, 9, 10] was

used to measure the magnetic dipole strength B(M1↑) at the High-Intensity

Gamma-Ray Source (HIγS) [11] of the Triangle Universities Nuclear Labo-

ratory (TUNL) [12]. Linearly polarized (99%) and quasi-monoenergetic 9.88

MeV photons were produced via Compton back-scattering of 543 MeV elec-

trons from 548 nm free-electron laser photons. After traveling 53 m in a

high-vacuum tube the photon beam passed through a 1.27 cm diameter and

15 cm long lead collimator located approximately 4 m upstream of the 40Ar

target. The energy was centered at 9.88 MeV, with approximately 300 keV

full width at half maximum. The average photon ﬂux during the approxi-

mate 36.5-hour 40Ar irradiation was 2×107γ/s. The 40Ar target consisted of

an 84 mm inner diameter vessel made of 1 mm thick aluminum covered by a

4 mm thick layer of carbon ﬁber to accommodate the 40Ar pressure of 307.1

atm.

A schematic of the experimental setup is shown in Fig. 1. Four High-

Purity Germanium (HPGe) detectors were used to record the scattered γ

rays from 40Ar. Two 60% eﬃcient HPGe detectors (relative to a 300 ×300

NaI detector) were placed in the horizontal plane at polar angle θ= 90o

on either side of the incident photon beam at azimuthal angle φ= 0oand

180oto record de-excitation γrays of M1 character, while a 100% eﬃcient

HPGe detector was positioned in the vertical plane at θ= 90obelow the

photon beam at φ= 270oto record de-excitation γrays of electric dipole

(E1) nature. A fourth, 60% eﬃcient HPGe detector was placed at θ=127o

and φ= 135oto check on unlikely de-excitation γrays of electric quadrupole

(E2) nature. The distance between the center of the argon container and the

front face of the HPGe detectors #1, #2, #3, and #4 (see Fig. 1) is 13.7 cm,

12.8 cm, 13.8 cm, and 11.4 cm, respectively. The detector diameters ranged

from 6.46 to 6.97 cm. A thin-walled, 238U-based ﬁssion-ionization chamber

was positioned downstream of the 40Ar cell to record the incident photon

ﬂux [13].

A typical HPGe detector spectrum obtained with the vertical detector in

the 9.4 to 10.2 MeV γ-ray energy range is shown in Fig. 2 (a). This spectrum

is dominated by the strong E1 transition from the decay of the 9849 keV

state in 40Ar. Fig. 2 (a) also shows a normalized γ-ray spectrum measured

with the ﬁlled 40Ar cell replaced by an empty one, clearly indicating that

there are no interfering γ-ray transitions in the region of interest resulting

from the aluminum and/or carbon ﬁber of the containment vessel, the lead

collimators, and the environment. Fig. 2 (b) presents the γ-ray spectrum

recorded with one of the two horizontal HPGe detectors. This spectrum

features M1 transitions in 40Ar and is richer than the associated E1 spectrum

presented in Fig. 2 (a). The known M1 transition at 9757 keV is clearly seen.

However, there is also a comparably strong γ-ray transition at 9840 keV, and

weaker γ-ray lines originating from levels in 40Ar at 9697.5, 9805.6, 9871.7,

9893.9, 10020.5 and 10033.9 keV. A comparison of the 9840 keV γ-ray line

with others, including that at 9757 keV, shows that the line in question

is broader than those in its vicinity. According to Figs. 2 (a) and 2 (b)

Figure 1: Schematic of experimental setup. The linearly polarized (in the horizontal plane)

photon beam enters from the left side and passes through an 40Ar ﬁlled container, which

is viewed by four HPGe detectors, the angles of which are given in the text. The photon

ﬂux is monitored by the 238U ﬁssion chamber shown on the right side.

the energy of the very strong E1 transition of 9849 keV almost coincides

with that of the M1 transition at 9.840 MeV. Inspecting Fig. 2 of Li et al.

[4] we note that the new M1 transition at 9840 keV is also seen in this

work, but without mention in the text. Monte-Carlo calculations performed

for the present geometrical arrangement of the 40Ar target and the HPGe

detectors predict that our measured M1 spectrum includes a contamination

of approximately 3% of the E1 yield, resulting in a 23% reduction of the M1

strength at 9849 keV. This conclusion is in agreement with a two-Gaussian

ﬁt attempt to the line shape recorded at 9849 keV, as can be seen from

Fig. 3. Focusing on the energy calibration of our HPGe detectors, we note

that natural background lines, and γ-ray test sources, including 56Co, were

used to cover the energy range up to 3.45 MeV. Beyond this energy, known

γ-ray transitions in 27Al excited by the incident photon beam were used to

extend the energy calibration up to 10 MeV. This procedure provided results

consistent with previous energy assignments for states in 40Ar.

The normalized 40Ar-out spectra were subtracted from the 40Ar-in spec-

tra, with the normalization factor deduced from the yields recorded with the

238U based ﬁssion chamber referred to earlier.

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Neutral-currentneutrinocrosssectionandexpectedsupernovasignalsfor40Arfromathree-foldincreaseinthemagneticdipolestrengthW.Tornowa,b,A.P.Tonchevc,a,S.W.Fincha,b,Krishichayana,b,X.B.Wangd,A.C.Hayese,H.G.D.Yeomanse,f,D.A.Newmarke,gaDepartmentofPhysicsDukeUniversityDurhamNC27708-0308USAbTriangleUniversit...

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Neutral-current neutrino cross section and expected supernova signals for 40Ar from a three-fold increase in the magnetic dipole strength

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