<p>The <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\upbeta\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">β</mi> </math></EquationSource> </InlineEquation>-delayed neutron emission probability (<InlineEquation ID="IEq7"> <EquationSource Format="TEX">\({P}_{\text {n}}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mtext>n</mtext> </msub> </math></EquationSource> </InlineEquation>) is a key observable for characterizing the decay strength of very neutron-rich nuclei and the rapid neutron capture process in nuclear astrophysics. A Long Helium-3 Neutron Array (LHENA) has been developed at the Beijing Rare Isotope Facility (BRIF) to enable <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\({P}_{\text {n}}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mtext>n</mtext> </msub> </math></EquationSource> </InlineEquation> measurements using Isotope&#xa0;Separator On&#xa0;Line (ISOL) pulsed beams. LHENA is designed to work in conjunction with a tape driver and auxiliary detectors, so that <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\upbeta\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">β</mi> </math></EquationSource> </InlineEquation> particles, <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\upbeta\)</EquationSource> <EquationSource Format="MATHML"><math> <mi mathvariant="normal">β</mi> </math></EquationSource> </InlineEquation>-delayed neutrons and <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\gamma\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>γ</mi> </math></EquationSource> </InlineEquation> rays emitted from the implanted nuclei can be measured simultaneously in cyclic mode. LHENA consists of 21 long <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(^{3}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow /> <mrow /> <mn>3</mn> </mmultiscripts> </math></EquationSource> </InlineEquation>He proportional counters embedded in a polyethylene moderator with a two-ring configuration, which provides a flat neutron detection efficiency up to 3 MeV according to our Geant4 simulations. The detection efficiency has been experimentally determined to be 16.4(<InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\pm 0.4\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>±</mo> <mn>0.4</mn> </mrow> </math></EquationSource> </InlineEquation>)% using the <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(^{51}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow /> <mrow /> <mn>51</mn> </mmultiscripts> </math></EquationSource> </InlineEquation>V(p,n)<InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(^{51}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow /> <mrow /> <mn>51</mn> </mmultiscripts> </math></EquationSource> </InlineEquation>Cr reaction for neutron energies in the 120–700&#xa0;keV range. A good efficiency flatness and a very low background have been verified for LHENA, laying a solid foundation for the first <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\({P}_{\text {n}}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>P</mi> <mtext>n</mtext> </msub> </math></EquationSource> </InlineEquation> measurement using very neutron-rich Rb isotopes at BRIF.</p>