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A long-time chopper for direct measurement of 4He(12C,16O)γ reaction cross section
Nuclear Physics A 758 (2005) 407c–410c
A long-time chopper for direct measurement of 4 He(12 C,16 O)γ reaction cross section H. Obaa∗ , K. Sagaraa , T. Shimizua , M. Oshiroa , T. Maedaa , and N. Ikedab a
Department of Physics, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, Japan b
Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, Japan Direct measurement of the 4 He(12 C,16 O)γ reaction cross section by detecting 16 O recoils is in progress at Kyushu University tandem laboratory (KUTL). A long-time chopper (LTC) has been developed to reject backgrounds from 16 O recoils which have time (energy) spread. In the measurement of 4 He(12 C,16 O)γ cross section at Ecm = 2.4 MeV, LTC was found to reduce the backgrounds by about three orders of magnitude. 1. Introduction
In the helium burning in massive stars the 4 He + 12 C → 16 O + γ reaction takes a very important role [1]. The reaction cross section (astrophysical S-factor) at around the stellar energy of Ecm ∼ 0.3 MeV is largely influenced by levels of 16 O just below the threshold, and we need cross section data at energies as low as possible to estimate the S-factor at 0.3 MeV. We have a plan to measure the cross section down to Ecm = 0.7 MeV, where the cross section is about 1 pbarn at Kyushu University tandem laboratory (KUTL) [2]. From the reaction of 4 He(12 C,16 O)γ, 16 O particles are recoiled out at forward angles within 2◦ . We detect all the 16 O recoils in one charge state. The 16 O recoils and the 12 C beam are separated using a recoil mass separator (RMS) shown in Fig. 1. RMS greatly reduces background 12 C particles. However, since the number of 16 O recoils is
r = 1.0 m, 30.0 deg Long-time Chopper Q3 Q4 D2 D1
Brow-in type Windowless MQ 4He gas target r = 2.5 m, 12.2 deg Beam ED Q1
SX1
F2
SX2 F1
Q2
r = 1.0 m, 30.0 deg
Figure 1. Recoil mass separator (RMS) and a long-time chopper (LTC) at KUTL. ∗
E-mail address: [email protected]
0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2005.05.074
408c
H. Oba et al. / Nuclear Physics A 758 (2005) 407c–410c
so small, background (BG) reduction by RMS is insufficient. Then, we transformed the tandem beam into a pulsed beam using a beam buncher and beam choppers, and times of flight (TOF) of particles were measured. BG/beam, which was defined as the ratio of the number of BG in the 16 O detection area to the number of the beam particles, was reduced from 10−11 to 10−14 by the TOF technique. In order to perform the measurement at Ecm = 0.7 MeV with our system, BG/beam should be less than 10−19 . Reduction of a beam halo by fine-tuning of buffer-slit positions, for example, may reduce BG level by 1-2 orders of magnitude. To drastically reduce BG, a new powerful instrument is necessary. We then have developed a long-time chopper (LTC) so as to pass 16 O recoils and to reject 12 C BG. LTC was installed in RMS. 2. Design of a long-time chopper (LTC) 2.1. Chopper for reaction recoils In general a beam chopper consists of an electric deflector and a slit. A sine-wave voltage is imposed on the deflector, and a beam can pass the slit at the time when the deflector voltage is effectively zero. A time width and a period of a pulsed beam are typically a few ns and 150 ns, respectively. A sine-wave voltage is suitable for a chopper of such a short-width pulsed beam. Recoiled particles from a reaction are spread in their energy hence are spread in their flight time. For example, 16 O recoils from 4 He(12 C,16 O)γ reaction at Ecm = 2.4 MeV have energy in the range of 7.2 ± 0.3 MeV and have spread of ±9 ns in their flight time at 4 m downstream from the target. A chopper for reaction recoils should have a finite time during which the deflector voltage is effectively zero. For the above example, the zero-voltage duration is about 12% of the period. Therefore, for a chopper for reaction recoils, (1) the voltage should be effectively zero during a time longer than 12% of a period to pass reaction recoils without deflection, and (2) the voltage should be high enough in other time to reject BG. 2.2. Flat-top voltage We decided to prepare such a voltage by adding the following three kinds of voltage; (a) a sine-wave voltage of frequency f1 and amplitude ±V1 , (b) a sine-wave voltage of frequency f2 = 3f1 and amplitude V2 = ±V1 /9 and (c) a static voltage of V3 = V1 − V2 . By
50
Voltage [kV]
f0 = 2 ~ 7 MHz V1 = ± 30 kV
V1
16O
passage ~ 18 ns
30
3f0 = 6 ~ 21 MHz V2 = ± 5 kV
f1 = 6.1 MHz
164 ns 40
BG(12C)
20
V3
10
16O
recoil
0 0
50
100
150
200
250
300
V3 = 40 kV
Time [ns]
Figure 2. Flat-top voltage with 0 V bottom.
Figure 3. Two deflectors and a slit of a long-time chopper for reaction recoils.
H. Oba et al. / Nuclear Physics A 758 (2005) 407c–410c
409c
500
TOF [ns]
450
400
350
16O
300
beam pulse interval (164 ns)
recoils
BG(12C)
250
200 2
3
4
5
6
7
8
9
10
Energy [MeV]
Figure 4. 12 C BG that passed LTC in the same time as 16 O recoils come to the detector in different times and 12 C BG are completely rejected in an TOF-energy spectrum. adding (a) and (b), an effectively flat-top shape voltage of frequency f1 and of amplitude ±(8/9)V1 is obtained. The duration of a flat-top is about 20% of the period. By further addition of (c), one of the flat-top voltages is set to be zero, so as to pass reaction recoils through the chopper. Fig. 2 shows the flat-top shape voltage where f1 is 6.1 MHz and V1 is 25 kV. Rather high voltage is necessary because the distance between the deflector and the slit of chopper is short as 80 cm in our system (RMS). Since imposition of two voltages (a) and (b) on a deflector needs large electric power consumption, we use two succeeding deflectors on which voltages (a) and (b) are imposed separately, as shown in Fig. 3. The voltage (a) and (b), and the sine-wave voltages for the beam buncher and the beam choppers were synchronized. 2.3. Separation of BG We call the chopper with flat-top voltage as a long-time chopper (LTC). LTC was installed between the two magnetic deflectors (D1 and D2) in RMS as shown in Fig. 1. LTC deflects BG in the vertical direction. Although paths of 16 O recoils are slightly shifted vertically by the deflection in opposite directions in two deflectors, the shifts are negligibly small. LTC rejects BG in two ways. First, in the time when the deflector voltage is not at the flat bottom of 0 V, BG are deflected away and can not come to the detector. Second, in the time when the deflector voltage is at the flat bottom, both 16 O recoils and BG pass LTC. Some of BG may have the same magnetic rigidity as 16 O recoils and come to the detector. By the detector we measure the energy and the arriving time of the particles, with which 16 O recoils and BG are separated. Most BG are 12 C. If 12 C BG and 16 O recoils have the same energy and pass LTC at the same time, 12 C BG reach the detector first. In the two-dimensional spectrum for the energy and TOF, 12 C BG which passed LTC form bands as seen in Fig. 4. Since the bands are well separated from the area of 16 O recoils, 12 C BG are completely rejected in principle. 3. Manufacture of LTC The sine-wave voltages for LTC were produced by LC resonating circuits. The capacity C consisted of the capacity of the deflector itself and the capacity between the feed through of the high voltage and the wall of the vacuum chamber at the earth level. The inductance
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H. Oba et al. / Nuclear Physics A 758 (2005) 407c–410c
Figure 5. TOF vs. energy spectra for 4 He(12 C,16 O)γ experiment at Ecm = 2.4 MeV, taken without LTC (left) and with LTC (right), in 4 hours and 70 hours, respectively.
L was supplied by a coil. The coil was made of a hollow conductor whose temperature was controlled within ±0.5◦ by a cooling system so as to keep the inductance constant. Teflon was adopted for the insulating material for the feed through, because other materials such as acryl, Delrin and polycarbonate were heated up or induced surface discharge when a voltage of ±25 kV at 6.1 MHz was imposed. 4. Performance of LTC Using LTC, measurement of 4 He(12 C,16 O)γ reaction cross section at Ecm = 2.4 MeV was made at KUTL. Fig. 5 show the spectra taken without LTC (left) and with LTC (right), in 4 hours and 70 hours, respectively. By the use of LTC, BG in the 16 O detection area (circle) were entirely reduced and 16 O events were clearly detected. In this experiment LTC was found to reduce BG by about three orders of magnitude. Details of the experiment are reported in this symposium [3]. In another test experiment at high BG level, LTC reduced BG by 4-5 orders of magnitude. 5. Summary A long-time chopper (LTC) has been developed in order to reduce backgrounds (BG) in He(12 C,16 O)γ experiment. LTC is operated with a flat-top high-frequency high voltage having a flat-bottom of 0 V, during which 16 O recoils pass through LTC. LTC was found to have a power to reduce BG by about 3 orders of magnitude. To our knowledge, there has been no LTC device made previously. 4
REFERENCES 1. W.A. Fowler, Rev. Mod. Phys. 56 (1984) 149. 2. N. Ikeda et al., Nucl. Phys. A 718 (2003) 558c. 3. K. Sagara et al., “Direct measurement of 4 He(12 C,16 O)γ reaction cross section around Ecm = 2.4 MeV at KUTL”, NIC8 Symposium (2004)
A long-time chopper for direct measurement of 4 He(12 C,16 O)γ reaction cross section H. Obaa∗ , K. Sagaraa , T. Shimizua , M. Oshiroa , T. Maedaa , and N. Ikedab a
Department of Physics, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, Japan b
Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, Japan Direct measurement of the 4 He(12 C,16 O)γ reaction cross section by detecting 16 O recoils is in progress at Kyushu University tandem laboratory (KUTL). A long-time chopper (LTC) has been developed to reject backgrounds from 16 O recoils which have time (energy) spread. In the measurement of 4 He(12 C,16 O)γ cross section at Ecm = 2.4 MeV, LTC was found to reduce the backgrounds by about three orders of magnitude. 1. Introduction
In the helium burning in massive stars the 4 He + 12 C → 16 O + γ reaction takes a very important role [1]. The reaction cross section (astrophysical S-factor) at around the stellar energy of Ecm ∼ 0.3 MeV is largely influenced by levels of 16 O just below the threshold, and we need cross section data at energies as low as possible to estimate the S-factor at 0.3 MeV. We have a plan to measure the cross section down to Ecm = 0.7 MeV, where the cross section is about 1 pbarn at Kyushu University tandem laboratory (KUTL) [2]. From the reaction of 4 He(12 C,16 O)γ, 16 O particles are recoiled out at forward angles within 2◦ . We detect all the 16 O recoils in one charge state. The 16 O recoils and the 12 C beam are separated using a recoil mass separator (RMS) shown in Fig. 1. RMS greatly reduces background 12 C particles. However, since the number of 16 O recoils is
r = 1.0 m, 30.0 deg Long-time Chopper Q3 Q4 D2 D1
Brow-in type Windowless MQ 4He gas target r = 2.5 m, 12.2 deg Beam ED Q1
SX1
F2
SX2 F1
Q2
r = 1.0 m, 30.0 deg
Figure 1. Recoil mass separator (RMS) and a long-time chopper (LTC) at KUTL. ∗
E-mail address: [email protected]
0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2005.05.074
408c
H. Oba et al. / Nuclear Physics A 758 (2005) 407c–410c
so small, background (BG) reduction by RMS is insufficient. Then, we transformed the tandem beam into a pulsed beam using a beam buncher and beam choppers, and times of flight (TOF) of particles were measured. BG/beam, which was defined as the ratio of the number of BG in the 16 O detection area to the number of the beam particles, was reduced from 10−11 to 10−14 by the TOF technique. In order to perform the measurement at Ecm = 0.7 MeV with our system, BG/beam should be less than 10−19 . Reduction of a beam halo by fine-tuning of buffer-slit positions, for example, may reduce BG level by 1-2 orders of magnitude. To drastically reduce BG, a new powerful instrument is necessary. We then have developed a long-time chopper (LTC) so as to pass 16 O recoils and to reject 12 C BG. LTC was installed in RMS. 2. Design of a long-time chopper (LTC) 2.1. Chopper for reaction recoils In general a beam chopper consists of an electric deflector and a slit. A sine-wave voltage is imposed on the deflector, and a beam can pass the slit at the time when the deflector voltage is effectively zero. A time width and a period of a pulsed beam are typically a few ns and 150 ns, respectively. A sine-wave voltage is suitable for a chopper of such a short-width pulsed beam. Recoiled particles from a reaction are spread in their energy hence are spread in their flight time. For example, 16 O recoils from 4 He(12 C,16 O)γ reaction at Ecm = 2.4 MeV have energy in the range of 7.2 ± 0.3 MeV and have spread of ±9 ns in their flight time at 4 m downstream from the target. A chopper for reaction recoils should have a finite time during which the deflector voltage is effectively zero. For the above example, the zero-voltage duration is about 12% of the period. Therefore, for a chopper for reaction recoils, (1) the voltage should be effectively zero during a time longer than 12% of a period to pass reaction recoils without deflection, and (2) the voltage should be high enough in other time to reject BG. 2.2. Flat-top voltage We decided to prepare such a voltage by adding the following three kinds of voltage; (a) a sine-wave voltage of frequency f1 and amplitude ±V1 , (b) a sine-wave voltage of frequency f2 = 3f1 and amplitude V2 = ±V1 /9 and (c) a static voltage of V3 = V1 − V2 . By
50
Voltage [kV]
f0 = 2 ~ 7 MHz V1 = ± 30 kV
V1
16O
passage ~ 18 ns
30
3f0 = 6 ~ 21 MHz V2 = ± 5 kV
f1 = 6.1 MHz
164 ns 40
BG(12C)
20
V3
10
16O
recoil
0 0
50
100
150
200
250
300
V3 = 40 kV
Time [ns]
Figure 2. Flat-top voltage with 0 V bottom.
Figure 3. Two deflectors and a slit of a long-time chopper for reaction recoils.
H. Oba et al. / Nuclear Physics A 758 (2005) 407c–410c
409c
500
TOF [ns]
450
400
350
16O
300
beam pulse interval (164 ns)
recoils
BG(12C)
250
200 2
3
4
5
6
7
8
9
10
Energy [MeV]
Figure 4. 12 C BG that passed LTC in the same time as 16 O recoils come to the detector in different times and 12 C BG are completely rejected in an TOF-energy spectrum. adding (a) and (b), an effectively flat-top shape voltage of frequency f1 and of amplitude ±(8/9)V1 is obtained. The duration of a flat-top is about 20% of the period. By further addition of (c), one of the flat-top voltages is set to be zero, so as to pass reaction recoils through the chopper. Fig. 2 shows the flat-top shape voltage where f1 is 6.1 MHz and V1 is 25 kV. Rather high voltage is necessary because the distance between the deflector and the slit of chopper is short as 80 cm in our system (RMS). Since imposition of two voltages (a) and (b) on a deflector needs large electric power consumption, we use two succeeding deflectors on which voltages (a) and (b) are imposed separately, as shown in Fig. 3. The voltage (a) and (b), and the sine-wave voltages for the beam buncher and the beam choppers were synchronized. 2.3. Separation of BG We call the chopper with flat-top voltage as a long-time chopper (LTC). LTC was installed between the two magnetic deflectors (D1 and D2) in RMS as shown in Fig. 1. LTC deflects BG in the vertical direction. Although paths of 16 O recoils are slightly shifted vertically by the deflection in opposite directions in two deflectors, the shifts are negligibly small. LTC rejects BG in two ways. First, in the time when the deflector voltage is not at the flat bottom of 0 V, BG are deflected away and can not come to the detector. Second, in the time when the deflector voltage is at the flat bottom, both 16 O recoils and BG pass LTC. Some of BG may have the same magnetic rigidity as 16 O recoils and come to the detector. By the detector we measure the energy and the arriving time of the particles, with which 16 O recoils and BG are separated. Most BG are 12 C. If 12 C BG and 16 O recoils have the same energy and pass LTC at the same time, 12 C BG reach the detector first. In the two-dimensional spectrum for the energy and TOF, 12 C BG which passed LTC form bands as seen in Fig. 4. Since the bands are well separated from the area of 16 O recoils, 12 C BG are completely rejected in principle. 3. Manufacture of LTC The sine-wave voltages for LTC were produced by LC resonating circuits. The capacity C consisted of the capacity of the deflector itself and the capacity between the feed through of the high voltage and the wall of the vacuum chamber at the earth level. The inductance
410c
H. Oba et al. / Nuclear Physics A 758 (2005) 407c–410c
Figure 5. TOF vs. energy spectra for 4 He(12 C,16 O)γ experiment at Ecm = 2.4 MeV, taken without LTC (left) and with LTC (right), in 4 hours and 70 hours, respectively.
L was supplied by a coil. The coil was made of a hollow conductor whose temperature was controlled within ±0.5◦ by a cooling system so as to keep the inductance constant. Teflon was adopted for the insulating material for the feed through, because other materials such as acryl, Delrin and polycarbonate were heated up or induced surface discharge when a voltage of ±25 kV at 6.1 MHz was imposed. 4. Performance of LTC Using LTC, measurement of 4 He(12 C,16 O)γ reaction cross section at Ecm = 2.4 MeV was made at KUTL. Fig. 5 show the spectra taken without LTC (left) and with LTC (right), in 4 hours and 70 hours, respectively. By the use of LTC, BG in the 16 O detection area (circle) were entirely reduced and 16 O events were clearly detected. In this experiment LTC was found to reduce BG by about three orders of magnitude. Details of the experiment are reported in this symposium [3]. In another test experiment at high BG level, LTC reduced BG by 4-5 orders of magnitude. 5. Summary A long-time chopper (LTC) has been developed in order to reduce backgrounds (BG) in He(12 C,16 O)γ experiment. LTC is operated with a flat-top high-frequency high voltage having a flat-bottom of 0 V, during which 16 O recoils pass through LTC. LTC was found to have a power to reduce BG by about 3 orders of magnitude. To our knowledge, there has been no LTC device made previously. 4
REFERENCES 1. W.A. Fowler, Rev. Mod. Phys. 56 (1984) 149. 2. N. Ikeda et al., Nucl. Phys. A 718 (2003) 558c. 3. K. Sagara et al., “Direct measurement of 4 He(12 C,16 O)γ reaction cross section around Ecm = 2.4 MeV at KUTL”, NIC8 Symposium (2004)