Preamble

A long He proportional counter array for the study of -delayed neutron emission probability at Qiang Wang, You-Bao Wang, Yu-Qiang Zhang, Ming-Hao Zhu, Jin-Long Ma, Jun-Wen Tian, Dong-Lin Xie, Hong-Wei Huang, Wei-Ping Lin, De-Hao Xie, Cun Wen, Yi Tang, Jun Su, Zhi-Wei Qin, Jun-Rui Ma, Wei-Ke Wan-Qin Tu, Wei Nan, Sheng-Quan Yan, Yun-Ju Li, Yang-Ping Shen, Bing Guo, and Wei-Ping Liu 1 China Institute of Atomic Energy, Beijing 102413, China Key Laboratory of Radiation Physics and Technology of the Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China Key Laboratory of Beam Technology of Ministry of Education, School of Physics and Astronomy, Beijing Normal University, Beijing 100875, China College of Science, Southern University of Science and Technology, Shenzhen 518055, China School of Physics, Xi’an Jiaotong University, Xi’an 710049, China -delayed neutron emission probability (P ) is an indispensable quantity to describe the decay strength of very neutron-rich nuclei and the rapid neutron capture process in nuclear astrophysics. A Long HElium-3 Neutron Array (LHENA), is developed at the Beijing Rare Isotope Facility (BRIF) to initiate P measurements using Isotope Separated On Line (ISOL) pulsed beams. LHENA is designed to work in conjunction with a tape driver and different detectors, so that particles, -delayed neutrons and rays emitted from the implanted nuclei can be measured simultaneously in periodical mode. LHENA consists of 21 long He proportional coun- ters embedded in a polyethylene-made moderator in two-ring structure, which allows for a flat neutron detection efficiency up to 3 MeV according to our Geant4 simulation. The detection efficiency has been experimentally determined to be 16.4( )% using the V(p,n) Cr reaction for neutron energies in the 120-700 keV range.

A good flatness in neutron detection efficiency and very low background of LHENA are verified, which lay a solid foundation for the first P measurement using very neutron-rich Rb isotopes at BRIF.

Keywords

He proportional counter, Long Helium-3 neutron array, -delayed neutron emission probability, Beijing Rare Isotope Facility (BRIF)

INTRODUCTION

-delayed neutron emission was first observed in 1939

accompanying with the neutron induced fission of uranium 3

and thorium isotopes [ ]. These neutrons are emitted in the decay process of neutron-rich fission fragments, when decay energy (Q ) is larger than the neutron separa-

tion energy (S n ) of the daughter nuclei, i.e. Q β 1 n = Q β - S n 7

0 . The β -delayed neutron emission probability P n is de- 8

fined by the fraction of the delayed neutron in the decay [ which demonstrates the fraction of strength above S . Al- though the average number of -delayed neutrons per fission

< µ d > is very small for fission systems of nuclear energy 12

application, the -delayed neutrons play a key role in safely controlling the prompt chain reaction at criticality in a nuclear power plant [ -delayed neutron groups from different fis- sion fragments were identified [ ], and a six-group fitting parameter set was established by Keepin and co-workers in

1957 [ 10 ]. To obtain < µ d > for different fission systems, 18

the summing method may be also applied besides direct mea- surement, by using the fission cumulative yield Y and the P

of each fragment, i.e. < µ d > = �

i Y i × P ni [ 11 ]. Up 21

Supported by the National Key Research and Development Project under grant No. 2022YFA1602301, the National Natural Science Foundation of China under Grants Nos. U2267205, 12275361 , the CNNC Fundamental Research Project and the Continuous-Support Basic Scientific Research Project. to date, there are large deviations between results from these two methods, even for the thermal neutron induced fission of 235 U, which is partly due to the large deviations of available data from different measurements. increases quickly with the increasing Q value, and -delayed multiple neutron emission channels become prob-

able if Q β xn > 0 for very neutron-rich nuclei [ 12 ].

In 28

this scenario, the decay property and nuclear structure are largely accessible only by the -delayed neutrons [ ]. In the rapid neutron capture process (r-process) of nuclear astro- physics [ ], the nucleosynthesis path runs close to the neutron drip line in the nuclide chart. A large number of delayed neutron emitters are involved, the delayed neutron emission modulates the heavy element abundance curve by altering the decay route [ ] and supplying fresh neutron source [ ]. Precise experimental data from delayed neutron emission represent a key input to r-process model calcula- tions.

Thousands of neutron-rich isotopes are predicted to be -delayed one or more neutron emitters, according to the present Atomic Mass Evaluation in 2020(AME2020) [ The quality of the existing experimental data is not sufficient

for various technical and scientific applications. It is there- 44

fore urgent to perform new measurements with leading-edge neutron detector arrays and high-quality neutron-rich beams, which become available at BRIF [ ] recently and at the High Intensity heavy-ion Accelerator Facility (HIAF) [ the near future. We report in this paper the installation of LHENA at BRIF for the study of -delayed neutron emission probability. The LHENA design and the overall P detection

system is described in Section 2. The determination of the LHENA detection efficiency is detailed in Section 3. Results are presented in Section 4, followed by a short discussion and summary in Section 5.

LHENA SETUP

Method

By definition, P n describes the fraction of the delayed neu- 58

trons in the decay, therefore it can be simply deduced by the following formula,

P n = N n N β × ε β ε n , (1) 61

where are the counts of detected neutrons and particles, respectively; refer to the detection ef- ficiencies of neutrons and particles, respectively.

If the half life of the nuclide is short enough comparing to the very weak beam intensity, the counts of the particle could be approximated by that of the implantation ion, which is use- ful in studying P of very rare isotopes produced at projectile fragmentation facility like HIAF. -delayed neutrons could be measured with the Time- Of-Flight (TOF) method in principle, to obtain their kine- matic energy information. However, this is only feasible for very few isotopes that can be produced with high intensity.

In most cases, proportional counter array with very high de- tection efficiency must be employed to compensate for the very low beam intensity of very neutron-rich nuclei. For such a purpose, a neutron detection system based on the He gas proportional counter has been very popular for its high sensi- tivity, low intrinsic activity and excellent durability [ It works by the thermal neutron capture reaction of n + keV at the maximum cross section of about 5330 barn. Such a high-efficiency He neutron counter array is free from cross-talk of multiple scattering, thus allows for neutron multiplicity measurements [ Normally, the P measurement is performed in the -n co- incidence mode [ ], which can be expressed as where is the counts of neutrons detected in coincidence particles, and represents the detection efficiency with energies corresponding to the neutron emission. When equals to , Equation is simplified as

P n = N β − n N β · ε n . (3) 92

By using -n coincidence, one avoids the precise deter- mination of . When the neutron counting rate is low, this method has the advantage to largely improve the signal-to- background ratio [ The measured and neutrons usually do not have clear energy information, it is necessary to combine HPGe detec- tors in the setup to enable characteristic rays measurement for nuclide identification. Furthermore, the detection ef- ficiency could be evaluated through the ray intensity ratio with/without the coincidence with particles [ ]. The neu- tron detection efficiency could be evaluated in a similar way with the help of the characteristic ray intensity.

LHENA description The schematic of the long He proportional counter ar- ray LHENA is shown in fig. . LHENA consists of 21 He proportional counters manufactured by GE-Reuter Stokes. The active length of the He proportional counter is 800.0(31.5) mm with a diameter of 25.4(0.8) mm. Each counter is filled with the He gas of 4 bar pressure, sealed with stainless steel housings. When a thermal neutron is cap- tured in the sensitive volume of the He gas, the energy re- leased from the reaction is converted into the kinetic ener- gies of the triton (191 keV) and the proton (574 keV). A full- energy peak of 765 keV is observed if both particles fully deposit their energies within the He gas volume. However,

due to the finite detector volume, either proton or triton may 118

hit the sealing wall and lose a part of its energy. This phe- nomenon is known as the wall effect [ ]. To reduce the wall effect and improve the peak to plateau ratio in the en- ergy spectrum, argon gas of 1.5 bar pressure is mixed in the 3 He proportional counter for its strong stopping power.

He proportional counters are embedded in a polyethylene-made neutron moderator. A central bore hole with a diameter of 10 cm is created in the moderator to ac- commodate the beam line and the target chamber. This com- pact configuration allows for a nearly solid angle coverage around the ion implantation position. The He proportional counters are distributed in two concentric rings. Three pro- portional counters are installed in the inner ring of 7.6 cm radius, and 18 proportional counters in the outer ring of 18 cm radius. This configuration enables a flat neutron detec- tion efficiency up to 2.5 MeV according to our Geant4 sim- ulation. The polyethylene moderator is surrounded with a 5 cm-thick 7% boronated polyethylene layer to shield the en- vironmental neutrons. The overall dimension of LHENA is As shown in fig. , the pulsed neutron-rich ISOL beam from BRIF is implanted in a tape, which is situated in the center of LHENA. The particles are measured with a sil- icon detector close to the tape in the vacuum chamber. The -delayed neutrons and rays are simultaneously measured with LHENA and a HPGe detector, respectively. To remove the long-lived isobaric contaminants and daughter nuclei, the measurement will be performed in a periodical mode, with a preset period of the radioactivity growth-in and decay ac- cording to the half-life of the nucleus in study. After each cycle, the tape is moved to a new position for a next-cycle ion

3 He counters

implantation. EXPERIMENT FOR LHENA EFFICIENCY CALIBRATION A typical neutron pulse-height spectrum measured by a 3 He proportional counter with an Am-Be neutron source is

shown in fig. 2 [FIGURE:2] . Signals from γ -rays, electronic noise, and 155

particles can be clearly distinguished from the neutron sig- nals in the spectrum. A test pulse of 1 Hz frequency from a

precise pulse generator is also recorded to monitor the system 158

operation and the dead time. The neutron signal of the He proportional counter exhibits long pulse feature, with a duration of about 500 µs and a rise

time of about 5 µs. This could lead to significant pulse pile- 162

up effect in high counting-rate experiment, which hinders the precise deduction of P value. The relationship between the pile-up rate and the counting rate is systematically studied by varying the position of the Am-Be neutron source in LHENA.

The pile-up rate increases linearly with the neutron counting rate as shown in fig. . The neutron counting rate for a single 3 He proportional counter should be kept low, below 120/s for example to make a marginal pile-up effect (less than 0.1%). neutron pulse signal He proportional counter. The relationship between the pile-up rate and the neutron counting rate is shown as an inset.

For the P measurement, it is very much desirable that the detection efficiency is flat for a wide neutron energy range.

Therefore, the detection efficiency and the energy dependence of LHENA must be known clearly.

For this purpose, the

51 V(p,n)

Cr is a widely-used reaction to calibrate the neu- tron arrays [ ] like LHENA. The neutron energy and intensity produced by the reaction vary slowly with angle, making it an ideal quasi-monochromatic and isotropic neu- tron source. In addition, Cr has a half life of 27.7 days and emits a 320 keV characteristic ray. The number of neutrons produced by the reaction could be determined by the activa- tion method with high accuracy, without having to rely on the reaction cross-section data. However, its application is lim-

ited to E p = 2330 keV, above this energy additional neutron 184

channel is open that corresponds to the first excited state of

51 Cr at

E x = 749 keV. 186

The experiment was carried out at the nuclear physics ex- periment (NPE) terminal of the 3-MV tandetron accelera-

tor [ 35 , 36 ] at Sichuan University. The 51 V target used in the 189

experiment was made by evaporating natural vanadium onto 1 mm thick tantalum with a diameter of 15 mm. The nomi- nal thickness of V is 100 corresponding to a 8 – 10

keV proton energy loss at E p = 1700 − 2300 keV. The 1 193

mm thick tantalum substrate ensures the mechanical strength 194

required for vacuum sealing. The experimental setup consists of stainless steel pipes with an outer diameter of 90 mm and a wall thickness of 3 mm, as shown in fig. . Inside, there is a coaxial copper cryotrap pipe with an outer diameter of 60 mm and a wall thickness of 2 mm, which was cooled by liq-

uid nitrogen to prevent carbon buildup on the target surface. 200

At the end of the cryotrap pipe, a 5 mm diameter tantalum collimator was installed. The negative voltage ring, with a diameter of 15 mm and a thickness of 2.5 mm, was fixed to

the collimator by insulated screws. A voltage of -600 V was applied during the experiment.

Cryotrap

3 He counters

Water Cooling Proton Beam target Collimator

600 V Electrode

V(p,n) Cr reaction. He proportional counters were operated at a bias volt- age of 1200 V, supplied by ORTEC 660 high-voltage power.

The signals from each proportional counter were processed by CAEN A1422 charge-sensitive preamplifier, which has a gain of 45 mV/MeV. The amplified signals were then ac- quired by the CAEN R5560 digital acquisition system, which has a sampling rate of 125 MS/s and a resolution of 14 bits.

The data was transmitted via USB to a computer for storage.

The accumulated charge on the target was measured using an ORTEC 439 digital current integrator.

Since the inner He proportional counters have much higher detection efficiency for low-energy neutrons, which re-

sult in significantly higher counter rates compared to the outer 218

ones. A CAEN DT5740D desktop digitizer system was used in combination with CoMPASS software to enable real-time

monitoring of the counting rates. Two inner-ring 3 He propor- 221

tional counters were monitored to avoid heavy pile-ups and 222

one outer-ring He counter for enough statistics. During the experiment, the counting rate for a single He proportional counter of the inner ring was kept to be below 120 counts/s, the overall counting rate of the LHENA system was below 500 counts/s.

The dead-time of the DAQ system is com- pletely negligible. We used a same V target for beam tun- ing, and replaced by a new V target for measurements. In

total, we made the measurements at E p = 1700, 1850, 2000, 230

2150, 2300 keV, corresponding to E n = 120 , 265, 410, 550, 231

and 695 keV, respectively. The number of neutrons emitted from the V(p,n) Cr re- action is equivalent to the number of the radioactive which decay through electron capture with a half-life of 27.7 days. The total number of emitted neutrons at each energy was therefore determined through the measurement of the 320 ray from the activated targets [ ]. The number of reactions ( ) occurring during the activation time( ) can be obtained using the following formula

N R = N γ ϵ 320 · η 320 · e λt w

where is the net counts of the 320 keV rays, is the counting time, is the waiting time elapsed between the end of the irradiation and the start of the counting, and is the decay constant of The neutron detection efficiency is simply calculated by

ϵ n = N n N R , (5) 247

where is the counts of the neutron recorded by LHENA.

The activated targets were measured by using the low- background Gamma spectrometer for Nuclear Activation Studies (GNAS) [ ] at CIAE, which consists of a well-type HPGe detector surrounded by optimized multi-layer shield- ing. GNAS has a near geometry for small radioactive sam- ples, thus is very efficient for radiochemical analysis and low- level -ray spectroscopy. A V target was irradiated with a high-flux proton beam in the experiment, the activity of was calibrated using a standard spectrometer [ ] at CIAE.

The absolute efficiency of GNAS was then determined to be 34.2%(1.2%) for the 320 keV -ray.

A typical spectrum of the Cr sample measured by GNAS is shown in fig. spectrum of Cr measured by GNAS.

The background neutron counting rate in the experimen- tal hall is 138(0 /s, which is negligible comparing to about 500 neutrons/s detected by LHENA. The neutron back- ground originated from the proton bombardment of the tanta- lum backing was also evaluated by comparing the normalized neutron yields under beam focusing and de-focusing condi-

tions at E p = 1 . 7 MeV. The introduced uncertainty was esti- 268

mated to be less than 0.5%.

RESULTS

LHENA position sensitivity along the beam axis was stud-

ied using the 51 V(p,n) 51 Cr reaction at E p = 2 MeV at 272

thirteen positions with Z = 0 , ± 3 , ± 6 , ± 10 , ± 15 , ± 20 , and 273

± 25 cm , respectively. The position with Z = 0 corresponds 274

to the target aligned in the center of LHENA. Positive values indicate the forward displacement of LHENA along the beam direction, while negative values represent the back- ward displacements. The relative efficiencies at different po-

sitions were normalized to the value at Z = 0 using the inte- 279

grated incident beam currents on the target. The experimental

and simulation data are normalized to 1 at Z = 0 , as shown 281

in fig. . Excellent agreement was shown between experi- mental data and simulations. The displacement of the neu- tron source within cm has negligible impact on the rela- tive efficiency, which is primarily benefited from the extended sensitive region of the He proportional counters. About 5% efficiency asymmetry was observed in the displacement range, which might attribute to the structural asymmetry of the polyethylene moderator and/or the He pro- portional counters. Anyway, this small asymmetry does not have any impact on P experiments, where the radioactiv- ity implantation occurs in the center region of LHENA that should not go beyond cm range. Furthermore, bench- mark measurement will be carried out with the same LHENA geometry for nuclei of well-known P values.

Experiment Data Simulation Data Relative Efficiency Position (cm) The experimental neutron detection efficiency of LHENA is listed in Table1, and shown in fig. , in comparison with Geant4 simulations. The experimental data show an average efficiency of 16.4( 0.4)% across five measurement points. A good flatness in neutron detection efficiency was verified.

The Geant4 simulation[ ] was performed indepen- dently for each ring as shown in fig. , which agrees rea- sonably well with the trend of the experimental data. The agreement is little worse for the outer ring at the lowest neu- tron energies. According to Geant4 simulation, the total de- tection efficiency of LHENA remains nearly constant within 0-3 MeV energy range; a marginal decrease of only 1.2% at 5 MeV. The good flatness of LHENA detection efficiency over wide neutron energies make the P measurement feasible for a large number of neutron-rich isotopes at BRIF.

As shown in fig. , our Geant4 simulated efficiencies exhibit a systematic overestimation of approximately 6%. In

Due to the reaction kinematics and finite target thickness, neutrons are not strictly mono-energetic. The average energy and the mini- mum and maximum energies are given.

(keV) (keV) Inter Outer Total Total Scaled Total Scaled Inter Scaled Outer Total Inter ring Outer ring Efficiency (%)

E n =550 keV, for the total efficiency and inter ring efficiency are 0.724 and 0.848, respectively. The scaled outer ring efficiency is the difference of scaled total and inner ring efficiencies.

order to compare the agreement of trend, a scale factor was applied from the ratio of experimental-to-simulated efficien-

cies at E n =0.55 MeV. Different scaling factors were em- 315

ployed for the inter ring and the total efficiencies. The simu- lation results follow very well the energy dependence of the measured data after applying the scaling factors.

DISCUSSION AND SUMMARY Neutron detector arrays based on the He proportional counters are widely used in many applications that require high-efficiency neutron detection. Although very large neu- tron energy range may be involved, in or charge particle induced neutron generation reaction study for example, a flat detection efficiency curve is a big advantage to obtain the absolute neutron yields regardless of their energies. Since mono-energetic neutron sources are very scarce, the determi- nation of the detection efficiency of such neutron detector ar-

rays largely rely on the simulations, which is one of the major sources for systematic errors of obtained data.

As shown in this work, the simulated results exhibit notable discrepancies in comparison with the experimental efficiency data. Such an overestimation has been widely observed for similar neutron arrays [ ], which demonstrate the importance of the experimental verification. The reason for the overestimation of detection efficiency in the simulation is not clear. Different corrections were arbitrarily adopted in the simulations including altering the geometry and den- sity of the polyethylene moderator, and the pressure of the

3 He counter [

Possibility of mixing of a little mount of boron in the polyethylene moderator is also discussed in Ref. [ ]. We have tested these presumptions in the simu- lation, it seems that none of these can give reasonable and conclusive solution. As a matter of fact, the neutron scatter- ing cross-sections employed in the simulations contain un- certainties, particularly in the modeling of molecular effects during neutron moderation within polyethylene and the inher- ent stochastic nature of neutron scattering. A larger deviation is indeed observed for the simulation of outer ring efficiency with longer neutron travel paths. Whether or not this discrep- ancy stems from the uncertainties in the neutron scattering cross-sections used in the simulations, it is important to ex- pand the experimental verification to higher neutron energy region.

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