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A new study of the 14N(p,γ)15O reaction at low energy
ELSEVIER
Nuclear
Physics
A719
(2003)
94c-98~ www.elsevier.com/locate/npe
A new study of t,he 14N(p, y)150 reaction
at low energy
A. Formicola”, H. Costantinib, G. Imbriani”, for the LUNA collaboration a Institut fiir Physik mit Ionenstrahlen, Ruhr-Universitat-Bochum, bUniversit& di Genova, Dipartimento
Bochum, Germany,
di Fisica and INFN, Genova, Italy
c Osservatorio Astronomico di Collurania, Teramo and INFN, Italy Precise knowledge of nuclear cross sections at stellar energies is of importance for cosmology as well as for nuclear astrophysics. Measuring these cross sections at the relevant energies is quite challenging not only for the extremely low cross sections involved but also because a lot of parameters like stopping powers and electron screening are not well known at low energies. The LUNA collaboration has installed a second high current accelerator of 400kV at the underground laboratory of Gran Sasso. It is dedicated to measure the cross section of charged particle induced nuclear reactions of astrophysical interest at energies far below the respective Coulomb barrier and possibly close or within the associated Gamow energy. 1. 14N(p, ?)I50 During most of its life, a low mass star burns H in the center via the pp chain. However, when the central H mass fraction reduces down to 0.1, the nuclear energy produced by the H-burning becomes not sufficient and the stellar core must contract to extract some energy from its gravitational field. Then, the central temperature (and the density) increases and the H-burning switches from the pp-chain to the more efficient CNO-burning. Thus, the escape from the main sequence is powered by the onset of the CNO burning, whose bottleneck is the 14N(p, -y)i50 reaction. A modification of the rate of this reaction alters the turn off luminosity, but, leaves almost unchanged the stellar lifetime, which is mainly determined by the rate of the pp reaction. The minimum energy explored in nuclear physics laboratories for this reaction is about 200 keV [l], well above the region of interest for the CNO burning in astrophysical condition (20 - 80 keV), so that the values used in stellar model computations are largely extrapolated. The most recent stellar models of low mass stars have been obtained by using the compilation of the nuclear reaction rates by Caughlan and Fowler (CF88 [2]). The r4N(p,y)i50 rate reported by the most recent compilation (NACRE collaboration [3]) does not substantially differ from the one tabulated by CF88. At solar energies the cross section of i4N(p, y)150 is dominated by a subthreshold resonance at -504 keV. Recently some indirect measurements of this subthreshold resonance have been performed employing the Doppler Shift Attenuation Method [5] and there are also some new calculations about its influence at solar energies [4]. At energies higher 0375.9474/03/$ - see front matter doi: 10.10 1 S/SO3759474(03)00974-6
8 2003 Elsevier
Science
B.V.
All rights
reserved.
A. Fornzicola et al. /Nuclear Physics A719 (2003) 94c-98~
95c
w+p -504 -J-+
Figure 1. Level structure of 150 near the proton threshold.
than 100 keV the cross section is dominated by the resonance at ER = 278 keV with transitions to the excited states at E, = 6793, 6176, 5183 keV, and the ground state in 150. According to Schriider et al. [l] that extended the measurements over a wide energy range, for laboratory energies EP = 0.2 to 3.6 MeV, the main contribution to the total S-factor at zero energy comes from transitions to the ground state in i50 and to the subthreshold state at E, = 6793 keV (Fig. 1). They found [l] for the total S-factor at zero energy S(0) = 3.20 & 0.54 keV-b. Angulo et al. [4] re-analyzed Schroder’s experimental data using a R-matrix model; they found for the total S factor at zero energy S(0) = 1.77 5 0.20 keV-b, which is a factor 1.7 lower than the values used in the recent compilations. The main difference concerns the S(0) factor for capture to the 150 ground state: they found a factor of 19 lower than the value of Schrijder (Fig. 2). We underline that the values at lower energies covered by Schroder are only upper limits, due to the strong presence of cosmic background in the spectrum. In summary, new measurements of the 14N(p, y)150 cross section at energies E 5 200 keV are necessary, in particular measurements of the transition to the ground state in 150. In order to investigate the very low energies of the 14N(p, y)150, the LUNA collaboration has installed an 400 kV underground accelerator facility at Laboratori Nazionali de1
96c
A. Formicola
et al. /Nuclear Physics A719 (2003) 94c-98~
Figure 2. The i4N(p, y)i50 capture data for transition to I50 ground state. The full curve represents the R-matrix fit of Angulo et al. [4], the dotted curve is the fit of Schrijder et al. [l].
Gran Sasso exploit.ing the strong suppression of cosmic background. Before the detailed 14N(p, y)150 mea surements, some preliminary tests were performed. Due to the strong dependence of the cross section from the energy it is important to have a precise knowledge of the absolute beam energy, the energy spread, and the stability of the beam. The absolute energy was determined to a precision of zt300 eV at E, = 130 to 400 keV using the energy of the capture y-ray transition of 12C(p,y)13N as well as the resonance energies at E, = 309, 338 and 389 keV of 23Na(p,y)24Mg, 26Mg(p,y)27A1, and 25Mg(p,y)26Alz respectively. The resonance studies led to a proton energy spread of better than 100 eV and a long term energy stability of 5 eV per hour. 2. MEASUREMENTS
OF l*N(p, y)150
The peculiarities of the 400 kV facility are particulary well suited for the study of 14N(p, y)150; where reaction y-ray lines up to 7.5 MeV have to be measured with very low intensities. High beam intensities and high detection resolutions have to be coupled to high target stability and purity, and thus low beam-induced background, as cosmic background is strongly suppressed and low intrinsic activity detectors are employed. Therefore, an extensive study of the quality of N targets has been performed, using implanted, evaporated, and sputtered targets. In all cases during the experiments targets were water-cooled directly on the backing. In the target chamber the target ladder was shadowed by a collimator, so that a uniform circular beam spot (a = 4 cm) was obtained within the target area by magnetic wobbling of the beam. In order to prevent build-up of impurities on the target, a LN-cooled copper cold finger was used. A Pb-shielded 120 % efficiency Ge detector was used in close geometry at 55” with respect to the beam direction. Repeated measurements of the resonance profile during long-term high power beam bombardments allow for monitoring of target quality and stability. We were able to conclude that sputtered targets of TiN on Ta backings have the most uniform number
A. Formicola
et al. /Nuclear
Physics A719 (2003) 94c-98~
97c
2.00
1.50
7
.
a.00
.
.
.
l
****
. % ‘F
0.50
0.00 0
2
4
dt
8
10
12
y
Figure
3. Stability
of thick
target
yield
300
after several hundreds
350
of FA (at Ep = 310 keV)
400
Proton Energy &[keV]
Figure
4. Thick
target
y-ray
yield
curve of the 278keV
resonance
of 14N(p, y)r50
density profile and withstand many days of beam bombardment with several hundreds of PA without any significant target deterioration (Fig. 3). The LUNA collaboration measured the S-factor in an energy range from 270 keV to 140 keV: Fig. 4 shows a thick target y yield curve at the 278 keV resonance illustrating that the r4N atoms are nearly homogeneously distributed from the surface to a depth of 115 keV at half maximum. All off resonance measurements are normalized to a yield point at the resonance plateau, in order to be independent of the effective stopping power. The range explored was covered using an energy step of 10 keV in order to apply for the analysis the differential method. Fig. 5 shows one of the differential y-ray spectrum obtained from the subtraction of two close energies, i.e. 270 keV and 260 keV. The analysis of the data in terms of a superposition of direct and resonant amplitudes is under course, as well as a new measurement of wy and branching ratios.
98c
A. Formicola
et al. /Nuclear
Physics A719 (2003) 94c-98~
3.OE-04 . spectrum --fit bg ‘-? Z.OE-04 z4 2i s E
l.OE-04
7400
7450
7500 Ey W-N
7550
7600
Figure 5. Differential spectrum between Ep= 270 and 260 keV for transition to I50 ground state. REFERENCES 1. 2. 3. 4. 5.
U.Schroder et al., Nucl. Phys. A 467 (1987) 240. Caughlan, G.R. & Fowler, W.A., Atomic and nuclear data tables 40 (1988) 283. C. Angulo et al., Nucl. Phys. A 656 (1999) 3. 6. Angulo et al., Nucl. Phys. A 690 (2001) 775. P.F.Bertone et al., Phys. Rev. Lett. 87 (2001) 155201.
Nuclear
Physics
A719
(2003)
94c-98~ www.elsevier.com/locate/npe
A new study of t,he 14N(p, y)150 reaction
at low energy
A. Formicola”, H. Costantinib, G. Imbriani”, for the LUNA collaboration a Institut fiir Physik mit Ionenstrahlen, Ruhr-Universitat-Bochum, bUniversit& di Genova, Dipartimento
Bochum, Germany,
di Fisica and INFN, Genova, Italy
c Osservatorio Astronomico di Collurania, Teramo and INFN, Italy Precise knowledge of nuclear cross sections at stellar energies is of importance for cosmology as well as for nuclear astrophysics. Measuring these cross sections at the relevant energies is quite challenging not only for the extremely low cross sections involved but also because a lot of parameters like stopping powers and electron screening are not well known at low energies. The LUNA collaboration has installed a second high current accelerator of 400kV at the underground laboratory of Gran Sasso. It is dedicated to measure the cross section of charged particle induced nuclear reactions of astrophysical interest at energies far below the respective Coulomb barrier and possibly close or within the associated Gamow energy. 1. 14N(p, ?)I50 During most of its life, a low mass star burns H in the center via the pp chain. However, when the central H mass fraction reduces down to 0.1, the nuclear energy produced by the H-burning becomes not sufficient and the stellar core must contract to extract some energy from its gravitational field. Then, the central temperature (and the density) increases and the H-burning switches from the pp-chain to the more efficient CNO-burning. Thus, the escape from the main sequence is powered by the onset of the CNO burning, whose bottleneck is the 14N(p, -y)i50 reaction. A modification of the rate of this reaction alters the turn off luminosity, but, leaves almost unchanged the stellar lifetime, which is mainly determined by the rate of the pp reaction. The minimum energy explored in nuclear physics laboratories for this reaction is about 200 keV [l], well above the region of interest for the CNO burning in astrophysical condition (20 - 80 keV), so that the values used in stellar model computations are largely extrapolated. The most recent stellar models of low mass stars have been obtained by using the compilation of the nuclear reaction rates by Caughlan and Fowler (CF88 [2]). The r4N(p,y)i50 rate reported by the most recent compilation (NACRE collaboration [3]) does not substantially differ from the one tabulated by CF88. At solar energies the cross section of i4N(p, y)150 is dominated by a subthreshold resonance at -504 keV. Recently some indirect measurements of this subthreshold resonance have been performed employing the Doppler Shift Attenuation Method [5] and there are also some new calculations about its influence at solar energies [4]. At energies higher 0375.9474/03/$ - see front matter doi: 10.10 1 S/SO3759474(03)00974-6
8 2003 Elsevier
Science
B.V.
All rights
reserved.
A. Fornzicola et al. /Nuclear Physics A719 (2003) 94c-98~
95c
w+p -504 -J-+
Figure 1. Level structure of 150 near the proton threshold.
than 100 keV the cross section is dominated by the resonance at ER = 278 keV with transitions to the excited states at E, = 6793, 6176, 5183 keV, and the ground state in 150. According to Schriider et al. [l] that extended the measurements over a wide energy range, for laboratory energies EP = 0.2 to 3.6 MeV, the main contribution to the total S-factor at zero energy comes from transitions to the ground state in i50 and to the subthreshold state at E, = 6793 keV (Fig. 1). They found [l] for the total S-factor at zero energy S(0) = 3.20 & 0.54 keV-b. Angulo et al. [4] re-analyzed Schroder’s experimental data using a R-matrix model; they found for the total S factor at zero energy S(0) = 1.77 5 0.20 keV-b, which is a factor 1.7 lower than the values used in the recent compilations. The main difference concerns the S(0) factor for capture to the 150 ground state: they found a factor of 19 lower than the value of Schrijder (Fig. 2). We underline that the values at lower energies covered by Schroder are only upper limits, due to the strong presence of cosmic background in the spectrum. In summary, new measurements of the 14N(p, y)150 cross section at energies E 5 200 keV are necessary, in particular measurements of the transition to the ground state in 150. In order to investigate the very low energies of the 14N(p, y)150, the LUNA collaboration has installed an 400 kV underground accelerator facility at Laboratori Nazionali de1
96c
A. Formicola
et al. /Nuclear Physics A719 (2003) 94c-98~
Figure 2. The i4N(p, y)i50 capture data for transition to I50 ground state. The full curve represents the R-matrix fit of Angulo et al. [4], the dotted curve is the fit of Schrijder et al. [l].
Gran Sasso exploit.ing the strong suppression of cosmic background. Before the detailed 14N(p, y)150 mea surements, some preliminary tests were performed. Due to the strong dependence of the cross section from the energy it is important to have a precise knowledge of the absolute beam energy, the energy spread, and the stability of the beam. The absolute energy was determined to a precision of zt300 eV at E, = 130 to 400 keV using the energy of the capture y-ray transition of 12C(p,y)13N as well as the resonance energies at E, = 309, 338 and 389 keV of 23Na(p,y)24Mg, 26Mg(p,y)27A1, and 25Mg(p,y)26Alz respectively. The resonance studies led to a proton energy spread of better than 100 eV and a long term energy stability of 5 eV per hour. 2. MEASUREMENTS
OF l*N(p, y)150
The peculiarities of the 400 kV facility are particulary well suited for the study of 14N(p, y)150; where reaction y-ray lines up to 7.5 MeV have to be measured with very low intensities. High beam intensities and high detection resolutions have to be coupled to high target stability and purity, and thus low beam-induced background, as cosmic background is strongly suppressed and low intrinsic activity detectors are employed. Therefore, an extensive study of the quality of N targets has been performed, using implanted, evaporated, and sputtered targets. In all cases during the experiments targets were water-cooled directly on the backing. In the target chamber the target ladder was shadowed by a collimator, so that a uniform circular beam spot (a = 4 cm) was obtained within the target area by magnetic wobbling of the beam. In order to prevent build-up of impurities on the target, a LN-cooled copper cold finger was used. A Pb-shielded 120 % efficiency Ge detector was used in close geometry at 55” with respect to the beam direction. Repeated measurements of the resonance profile during long-term high power beam bombardments allow for monitoring of target quality and stability. We were able to conclude that sputtered targets of TiN on Ta backings have the most uniform number
A. Formicola
et al. /Nuclear
Physics A719 (2003) 94c-98~
97c
2.00
1.50
7
.
a.00
.
.
.
l
****
. % ‘F
0.50
0.00 0
2
4
dt
8
10
12
y
Figure
3. Stability
of thick
target
yield
300
after several hundreds
350
of FA (at Ep = 310 keV)
400
Proton Energy &[keV]
Figure
4. Thick
target
y-ray
yield
curve of the 278keV
resonance
of 14N(p, y)r50
density profile and withstand many days of beam bombardment with several hundreds of PA without any significant target deterioration (Fig. 3). The LUNA collaboration measured the S-factor in an energy range from 270 keV to 140 keV: Fig. 4 shows a thick target y yield curve at the 278 keV resonance illustrating that the r4N atoms are nearly homogeneously distributed from the surface to a depth of 115 keV at half maximum. All off resonance measurements are normalized to a yield point at the resonance plateau, in order to be independent of the effective stopping power. The range explored was covered using an energy step of 10 keV in order to apply for the analysis the differential method. Fig. 5 shows one of the differential y-ray spectrum obtained from the subtraction of two close energies, i.e. 270 keV and 260 keV. The analysis of the data in terms of a superposition of direct and resonant amplitudes is under course, as well as a new measurement of wy and branching ratios.
98c
A. Formicola
et al. /Nuclear
Physics A719 (2003) 94c-98~
3.OE-04 . spectrum --fit bg ‘-? Z.OE-04 z4 2i s E
l.OE-04
7400
7450
7500 Ey W-N
7550
7600
Figure 5. Differential spectrum between Ep= 270 and 260 keV for transition to I50 ground state. REFERENCES 1. 2. 3. 4. 5.
U.Schroder et al., Nucl. Phys. A 467 (1987) 240. Caughlan, G.R. & Fowler, W.A., Atomic and nuclear data tables 40 (1988) 283. C. Angulo et al., Nucl. Phys. A 656 (1999) 3. 6. Angulo et al., Nucl. Phys. A 690 (2001) 775. P.F.Bertone et al., Phys. Rev. Lett. 87 (2001) 155201.