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The UGRA spectrometer for the measurement of the electric polarizability of the neutron
Nuclear Instruments and Methods in Physics Research A 440 (2000) 777}780
The UGRA spectrometer for the measurement of the electric polarizability of the neutron T.L. Enik*, R.V. Kharjuzov, L.V. Mitsyna, G.S. Samosvat Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna 141980, Russia
Abstract The new neutron spectrometer UGRA has been put into operation on the 250 m long time-of-#ight path of the IBR-30 booster in Dubna. It has been constructed for the determination of the electric polarizability of the neutron which will be derived from the precise measurement of the angular dependence of neutron scattering on heavy nuclei at energies &0.5}60 keV. The spectrometer is situated in a vacuum chamber of &3 m lateral dimensions, capable of holding up to 3 scattering samples and 16 3He-detectors (of &7 l volume each) in shielding tanks on a rotary platform. Some characteristics of the instrument are reported. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron electric polarizability; Di!erential scattering cross section; Heavy nuclei; 0.5}60 keV energy; 3He detectors
1. Introduction The neutron electric polarizability a is one of / the fundamental physical properties of the neutron which characterize its electromagnetic structure. It determines the induced electric dipole moment p"a e of the neutron exposed to an electric "eld e. / Close to a nucleus, the neutron senses the Coulomb "eld e of the nucleus which gives rise to an interaction <"a e2/2 in addition to the nuclear interac/ tion. There have been numerous attempts in the past to measure the electric polarizability of the neutron, but from the results one can only conclude that a lies within the interval &(0}2)]10~3 fm3, / neglecting the doubted [1}4] result given in Ref.
* Corresponding author.
[5]. All recent new a estimates are based on the / measurement of the neutron total cross section which has a weak negative contribution due to a which is proportional to E1@2 (E is the neutron / energy). The classical method to determine the electric polarizability of the neutron, by which the very "rst results on a were obtained, exploits the angular / dependence of elastic neutron scattering. The forward}backward asymmetry has a small contribution due to a which is proportional to E1@2. This / method resulted in the most precise a estimates in / its time: a (20]10~3 fm3 in 1959 [6] and / a (6]10~3 fm3 in 1966 [7]. The interest in reviv/ ing this method was expressed in 1983 at the workshop in Grenoble [8]. The decision to build the corresponding set-up was announced in 1990 [9], and the spectrometer UGRA (abbreviation of the Russian words `angular distributionsa) is now practically ready.
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 0 8 2 - 7
SECTION 6.
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T.L. Enik et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 777}780
2. Method The di!erential cross section for neutron scattering is given by p(0)"p /(4p)[1#u cos 0#u P (cos 0)] (1) 4 1 2 2 with the total scattering cross section p and the 4 anisotropy parameters u and u . The main chal1 2 lenge in measuring p(0) is to increase the accuracy of u , which determines the above-mentioned scat1 tering forward}backward asymmetry. In the mentioned prior experiments a precision of du +10~2 1 [6] and du +3]10~3 [7] was obtained. The di$1 culty is that there are di!erent contributions to u which are not easy to separate from the contri1 bution due to a : the main nuclear term propor/ tional to E but also a small term proportional to E3@2 which is due to the averaged e!ect of s- and p-wave resonances have to be taken into account. The algorithm for deriving a from the averaged / p(0) for a heavy nucleus is described in Ref. [9]. It consists of "tting three smooth curves to the experimental values of p , u and u in the energy range 4 1 2 &0.5}60 keV. Each curve is determined by eight constants, one of them being a . The other constants / are averaged neutron parameters for s- and p-wave, i.e. the potential scattering radii R@ and R@ , the 1 0 neutron strength functions S and S , with the latter 0 1 being de"ned by S "(S1@2#2S3@2)/3 and the radi1 1 1 ative strength functions Sc and Sc (see Refs. [9,10]). 1 0 The corresponding mathematical modelling includes data on average neutron parameters and total and radiative cross sections from other experiments and the analysis shows that it is possible to obtain a with an accuracy of (0.1}0.3)]10~3 fm3 corre/ sponding to the errors (2}3)]10~4 of u and u . 1 2 It is very important that, while the total cross section method is only applicable for the nucleus 208Pb, the discussed method can be applied for any heavy nucleus with good averaging over resonances and from which a massive elementary scattering sample can be prepared.
3. Spectrometer The UGRA spectrometer [11,12] is situated in a 3 m diameter and 3 m high aluminium chamber
with a common vacuum volume with entrance and exit neutron beam tubes 0.4 m in diameter (see Fig. 1). It is expected to work with up to 16 neutron detectors in shielding tanks "lled with para$n and B C which are rigidly mounted on a rotatable 4 platform. Each detector is a battery of industrial proportional counters or a big single counter with many wires which contains &7 l of 3He at the pressure 7}10 atm. The whole platform can be set at any of the 16 positions at 22.53 from one another, such that for each detector measurements at di!erent scattering angles from 253 up to 1553 can be performed. In the analysis only data from each single detector have to be related to each other, whereby a correction due to di!erent e$ciencies of the di!erent detectors is avoided. The neutron beam has a rectangular cross section of 12 (vertical) ]22 (horizontal) cm2. Rotation of the platform, selection of one of three scatterers and its angle with respect to the beam, data acquisition and their real-time analysis are accomplished by PC running a #exible code. The instrument works on the 250 m time-of#ight path of the Dubna booster IBR-30 [13,14] and has the following main characteristics. Its
Fig. 1. Arrangement of the UGRA spectrometer. (1) Chamber bottom; (2) middle section; (3,4) sleeves for connections with neutron beam-tubes; (5) bellows; (6) temperature compensator; (7) upper section; (8) membrane; (9) lower section; (10) rotary platform; (11) wheels for rotary platform; (12) neutron detector; (13) shielding tank; (14) adjustment device for centralisation; (15) vertical rod; (16) sample holder.
T.L. Enik et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 777}780
luminosity is determined by two parameters: the solid angle from the scatterer to one detector-battery is 0.014 sr and the absolute detection e$ciency is&0.6]E~0.2 (with E in keV). Due to rescattering in the shielding, the e$ciency decreases with increasing energy much more slowly than 1/v. The width of the time resolution function at half maximum is &6 ls which corresponds to the energy resolution *E+0.02E3@2 (*E and E in keV).
4. Spectrum First measurements have been performed which demonstrate the features of our instrument. As an example, a spectrum of the neutrons scattered o! a 3 mm thick scattering sample of metallic 238U is shown in Fig. 2. The experimental points are the di!erence of two spectra obtained with the sample in and out of the beam. Compared to the spectrum with the sample, the intensity in the background spectrum is only 0.1% (near channel 140, corresponding to&25 keV) and 1.4% (near channel 700, corresponding to &0.7 keV). Without sample, the main source of background is the detector's own
779
background (mainly cosmic radiation) which is &0.2 s~1 and constant in time. The data shown in the "gure were obtained after a measuring time of 15.5 h. Deep minima in the spectrum correspond to the `blacka resonances from 6 cm Al plate and 2.2 cm MnO powder in the beam. They allow the determination of the background from neutrons of `wronga energies (not corresponding to time-of#ight channels) by "tting to the minima with a proper polynomial. This background in the given channel has two components: neutrons of all energies from delayed neutrons in the booster and neutrons of higher energies which were "rst scattered by the scatterer and then reached the detector after they had experienced some scattering in the room. Both components increase as we approach small time-of#ight due to increase in the booster's neutron multiplication and the energy width of the time channels. In order to obtain the u and u values of the 1 2 cross section (1), it is su$cient to measure spectra analogous to those in Fig. 2 for three di!erent angles 0. The total scattering cross section p can be 4 obtained if similar measurements for a scatterer with known p are conducted (see details in Ref. 4 [11]). As for the errors du and du , they are of the 1 2 order of dN/N which is a relative error of counts in the chosen interval of the spectrum (under the condition that the values of dN/N for three 0 are approximately equal). For the spectrum shown in Fig. 2, dN/N is in the range 0.013}0.047, if it is divided into 14 intervals between 0.4 and 60 keV with an interval length of 15}150 channels depending on the energy.
5. Conclusions
Fig. 2. Neutron spectrum taken with one of the detectors during 15.5 h. The width of each channel is 1 ls, the scattering angle is 903, the angle between the uranium plate and the beam is 453.
A new neutron many-detector spectrometer has been built and tested at the 250 m time-of-#ight path of Dubna IBR-30 booster. The instrument allows one to measure with a high accuracy di!erential cross sections of scattering keV neutrons by nuclei and improves signi"cantly one of the old methods of measuring the neutron electric polarizability. Any heavy element, such as U, Th, Au, etc., which provides a good average through its resonances, can be used as a target for this purpose.
SECTION 6.
780
T.L. Enik et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 777}780
Acknowledgements The present work is supported in part by the research grant 97-02-16213 of Russian Foundation for Basic Research.
References [1] V.G. Nikolenko, A.B. Popov, JINR Communication E392-254, Dubna, 1992. [2] V.G. Nikolenko, A.B. Popov, Z. Phys. A 341 (1992) 365. [3] Yu.A. Alexandrov, Preprint of JINR E3-95-61, Dubna, 1995. [4] T.L. Enik, L.V. Mitsyna, V.G. Nikolenko, A.B. Popov, G.S. Samosvat, Yad. Fiz. 60 (1997) 648 (Phys. Atom. Nucl. 60 (1997) 567). [5] J. Schmiedmayer, P. Riehs, J.A. Harvey, N.W. Hill, Phys. Rev. Lett. 66 (1991) 1015. [6] R.M. Thaler, Phys. Rev. 114 (1959) 827.
[7] Yu.A. Alexandrov, G.S. Samosvat, Z. Sereeter, Tsoi Gen Sor. Pis'ma ZhETF 4 (1966) 196, (JETF Lett. 4 (1966) 134). [8] G.S. Samosvat, J. Phys. 45 (1984) C3}51. [9] Yu.A. Alexandrov, G.S. Samosvat, Proceedings of the VI International School on Neutron Physics, Alushta, 1990. JINR D3, 14-91-154, Dubna, 1991, p. 187 (in Russian). [10] L.V. Mitsyna, A.B. Popov, G.S. Samosvat, in: Proceedings of the International Conference on Nuclear Data for Science and Technology, Mito, Japan, JAERI, 1988, p. 111. [11] B.I. Voronov, T.L. Enik, V.A. Ermakov, V.I. Konstantinov, E.I. Litvinenko, L.V. Mitsyna, G.S. Samosvat, A.A. Smirnov, V.A. Trepalin, R.V. Kharjuzov, JINR Communication P13-97-36, Dubna, 1997 ( in Russian). [12] T.L. Enik, L.V. Mitsyna, G.S. Samosvat, A.A. Smirnov, R.V. Kharjuzov, JINR Communication P13-97-372, Dubna, 1997 (in Russian). [13] V.L. Aksenov, N.S. Dikansky, V.L. Lomidze, A.V. Novokhatsky, Yu.P. Popov, V.T. Rudenko, A.N. Skrinsky, W.I. Furman, JINR E3-92-110, Dubna, 1992. [14] V.V. Golikov, Zh.A. Kozlov, L.K. Kulkin, L.B. Pikelner, V.T. Rudenko, E.I. Sharapov. JINR 3-5736, Dubna, 1971 (in Russian).
The UGRA spectrometer for the measurement of the electric polarizability of the neutron T.L. Enik*, R.V. Kharjuzov, L.V. Mitsyna, G.S. Samosvat Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna 141980, Russia
Abstract The new neutron spectrometer UGRA has been put into operation on the 250 m long time-of-#ight path of the IBR-30 booster in Dubna. It has been constructed for the determination of the electric polarizability of the neutron which will be derived from the precise measurement of the angular dependence of neutron scattering on heavy nuclei at energies &0.5}60 keV. The spectrometer is situated in a vacuum chamber of &3 m lateral dimensions, capable of holding up to 3 scattering samples and 16 3He-detectors (of &7 l volume each) in shielding tanks on a rotary platform. Some characteristics of the instrument are reported. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron electric polarizability; Di!erential scattering cross section; Heavy nuclei; 0.5}60 keV energy; 3He detectors
1. Introduction The neutron electric polarizability a is one of / the fundamental physical properties of the neutron which characterize its electromagnetic structure. It determines the induced electric dipole moment p"a e of the neutron exposed to an electric "eld e. / Close to a nucleus, the neutron senses the Coulomb "eld e of the nucleus which gives rise to an interaction <"a e2/2 in addition to the nuclear interac/ tion. There have been numerous attempts in the past to measure the electric polarizability of the neutron, but from the results one can only conclude that a lies within the interval &(0}2)]10~3 fm3, / neglecting the doubted [1}4] result given in Ref.
* Corresponding author.
[5]. All recent new a estimates are based on the / measurement of the neutron total cross section which has a weak negative contribution due to a which is proportional to E1@2 (E is the neutron / energy). The classical method to determine the electric polarizability of the neutron, by which the very "rst results on a were obtained, exploits the angular / dependence of elastic neutron scattering. The forward}backward asymmetry has a small contribution due to a which is proportional to E1@2. This / method resulted in the most precise a estimates in / its time: a (20]10~3 fm3 in 1959 [6] and / a (6]10~3 fm3 in 1966 [7]. The interest in reviv/ ing this method was expressed in 1983 at the workshop in Grenoble [8]. The decision to build the corresponding set-up was announced in 1990 [9], and the spectrometer UGRA (abbreviation of the Russian words `angular distributionsa) is now practically ready.
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 0 8 2 - 7
SECTION 6.
778
T.L. Enik et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 777}780
2. Method The di!erential cross section for neutron scattering is given by p(0)"p /(4p)[1#u cos 0#u P (cos 0)] (1) 4 1 2 2 with the total scattering cross section p and the 4 anisotropy parameters u and u . The main chal1 2 lenge in measuring p(0) is to increase the accuracy of u , which determines the above-mentioned scat1 tering forward}backward asymmetry. In the mentioned prior experiments a precision of du +10~2 1 [6] and du +3]10~3 [7] was obtained. The di$1 culty is that there are di!erent contributions to u which are not easy to separate from the contri1 bution due to a : the main nuclear term propor/ tional to E but also a small term proportional to E3@2 which is due to the averaged e!ect of s- and p-wave resonances have to be taken into account. The algorithm for deriving a from the averaged / p(0) for a heavy nucleus is described in Ref. [9]. It consists of "tting three smooth curves to the experimental values of p , u and u in the energy range 4 1 2 &0.5}60 keV. Each curve is determined by eight constants, one of them being a . The other constants / are averaged neutron parameters for s- and p-wave, i.e. the potential scattering radii R@ and R@ , the 1 0 neutron strength functions S and S , with the latter 0 1 being de"ned by S "(S1@2#2S3@2)/3 and the radi1 1 1 ative strength functions Sc and Sc (see Refs. [9,10]). 1 0 The corresponding mathematical modelling includes data on average neutron parameters and total and radiative cross sections from other experiments and the analysis shows that it is possible to obtain a with an accuracy of (0.1}0.3)]10~3 fm3 corre/ sponding to the errors (2}3)]10~4 of u and u . 1 2 It is very important that, while the total cross section method is only applicable for the nucleus 208Pb, the discussed method can be applied for any heavy nucleus with good averaging over resonances and from which a massive elementary scattering sample can be prepared.
3. Spectrometer The UGRA spectrometer [11,12] is situated in a 3 m diameter and 3 m high aluminium chamber
with a common vacuum volume with entrance and exit neutron beam tubes 0.4 m in diameter (see Fig. 1). It is expected to work with up to 16 neutron detectors in shielding tanks "lled with para$n and B C which are rigidly mounted on a rotatable 4 platform. Each detector is a battery of industrial proportional counters or a big single counter with many wires which contains &7 l of 3He at the pressure 7}10 atm. The whole platform can be set at any of the 16 positions at 22.53 from one another, such that for each detector measurements at di!erent scattering angles from 253 up to 1553 can be performed. In the analysis only data from each single detector have to be related to each other, whereby a correction due to di!erent e$ciencies of the di!erent detectors is avoided. The neutron beam has a rectangular cross section of 12 (vertical) ]22 (horizontal) cm2. Rotation of the platform, selection of one of three scatterers and its angle with respect to the beam, data acquisition and their real-time analysis are accomplished by PC running a #exible code. The instrument works on the 250 m time-of#ight path of the Dubna booster IBR-30 [13,14] and has the following main characteristics. Its
Fig. 1. Arrangement of the UGRA spectrometer. (1) Chamber bottom; (2) middle section; (3,4) sleeves for connections with neutron beam-tubes; (5) bellows; (6) temperature compensator; (7) upper section; (8) membrane; (9) lower section; (10) rotary platform; (11) wheels for rotary platform; (12) neutron detector; (13) shielding tank; (14) adjustment device for centralisation; (15) vertical rod; (16) sample holder.
T.L. Enik et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 777}780
luminosity is determined by two parameters: the solid angle from the scatterer to one detector-battery is 0.014 sr and the absolute detection e$ciency is&0.6]E~0.2 (with E in keV). Due to rescattering in the shielding, the e$ciency decreases with increasing energy much more slowly than 1/v. The width of the time resolution function at half maximum is &6 ls which corresponds to the energy resolution *E+0.02E3@2 (*E and E in keV).
4. Spectrum First measurements have been performed which demonstrate the features of our instrument. As an example, a spectrum of the neutrons scattered o! a 3 mm thick scattering sample of metallic 238U is shown in Fig. 2. The experimental points are the di!erence of two spectra obtained with the sample in and out of the beam. Compared to the spectrum with the sample, the intensity in the background spectrum is only 0.1% (near channel 140, corresponding to&25 keV) and 1.4% (near channel 700, corresponding to &0.7 keV). Without sample, the main source of background is the detector's own
779
background (mainly cosmic radiation) which is &0.2 s~1 and constant in time. The data shown in the "gure were obtained after a measuring time of 15.5 h. Deep minima in the spectrum correspond to the `blacka resonances from 6 cm Al plate and 2.2 cm MnO powder in the beam. They allow the determination of the background from neutrons of `wronga energies (not corresponding to time-of#ight channels) by "tting to the minima with a proper polynomial. This background in the given channel has two components: neutrons of all energies from delayed neutrons in the booster and neutrons of higher energies which were "rst scattered by the scatterer and then reached the detector after they had experienced some scattering in the room. Both components increase as we approach small time-of#ight due to increase in the booster's neutron multiplication and the energy width of the time channels. In order to obtain the u and u values of the 1 2 cross section (1), it is su$cient to measure spectra analogous to those in Fig. 2 for three di!erent angles 0. The total scattering cross section p can be 4 obtained if similar measurements for a scatterer with known p are conducted (see details in Ref. 4 [11]). As for the errors du and du , they are of the 1 2 order of dN/N which is a relative error of counts in the chosen interval of the spectrum (under the condition that the values of dN/N for three 0 are approximately equal). For the spectrum shown in Fig. 2, dN/N is in the range 0.013}0.047, if it is divided into 14 intervals between 0.4 and 60 keV with an interval length of 15}150 channels depending on the energy.
5. Conclusions
Fig. 2. Neutron spectrum taken with one of the detectors during 15.5 h. The width of each channel is 1 ls, the scattering angle is 903, the angle between the uranium plate and the beam is 453.
A new neutron many-detector spectrometer has been built and tested at the 250 m time-of-#ight path of Dubna IBR-30 booster. The instrument allows one to measure with a high accuracy di!erential cross sections of scattering keV neutrons by nuclei and improves signi"cantly one of the old methods of measuring the neutron electric polarizability. Any heavy element, such as U, Th, Au, etc., which provides a good average through its resonances, can be used as a target for this purpose.
SECTION 6.
780
T.L. Enik et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 777}780
Acknowledgements The present work is supported in part by the research grant 97-02-16213 of Russian Foundation for Basic Research.
References [1] V.G. Nikolenko, A.B. Popov, JINR Communication E392-254, Dubna, 1992. [2] V.G. Nikolenko, A.B. Popov, Z. Phys. A 341 (1992) 365. [3] Yu.A. Alexandrov, Preprint of JINR E3-95-61, Dubna, 1995. [4] T.L. Enik, L.V. Mitsyna, V.G. Nikolenko, A.B. Popov, G.S. Samosvat, Yad. Fiz. 60 (1997) 648 (Phys. Atom. Nucl. 60 (1997) 567). [5] J. Schmiedmayer, P. Riehs, J.A. Harvey, N.W. Hill, Phys. Rev. Lett. 66 (1991) 1015. [6] R.M. Thaler, Phys. Rev. 114 (1959) 827.
[7] Yu.A. Alexandrov, G.S. Samosvat, Z. Sereeter, Tsoi Gen Sor. Pis'ma ZhETF 4 (1966) 196, (JETF Lett. 4 (1966) 134). [8] G.S. Samosvat, J. Phys. 45 (1984) C3}51. [9] Yu.A. Alexandrov, G.S. Samosvat, Proceedings of the VI International School on Neutron Physics, Alushta, 1990. JINR D3, 14-91-154, Dubna, 1991, p. 187 (in Russian). [10] L.V. Mitsyna, A.B. Popov, G.S. Samosvat, in: Proceedings of the International Conference on Nuclear Data for Science and Technology, Mito, Japan, JAERI, 1988, p. 111. [11] B.I. Voronov, T.L. Enik, V.A. Ermakov, V.I. Konstantinov, E.I. Litvinenko, L.V. Mitsyna, G.S. Samosvat, A.A. Smirnov, V.A. Trepalin, R.V. Kharjuzov, JINR Communication P13-97-36, Dubna, 1997 ( in Russian). [12] T.L. Enik, L.V. Mitsyna, G.S. Samosvat, A.A. Smirnov, R.V. Kharjuzov, JINR Communication P13-97-372, Dubna, 1997 (in Russian). [13] V.L. Aksenov, N.S. Dikansky, V.L. Lomidze, A.V. Novokhatsky, Yu.P. Popov, V.T. Rudenko, A.N. Skrinsky, W.I. Furman, JINR E3-92-110, Dubna, 1992. [14] V.V. Golikov, Zh.A. Kozlov, L.K. Kulkin, L.B. Pikelner, V.T. Rudenko, E.I. Sharapov. JINR 3-5736, Dubna, 1971 (in Russian).