- Email: [email protected]
Computing at the Dubna gas-filled recoil separator
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332 www.elsevier.com/locate/nima
Computing at the Dubna gas-filled recoil separator Yuri S. Tsyganov, Alexandr N. Polyakov Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russian Federation Available online 1 December 2005
Abstract Simulation codes for the spectra of heavy implanted nuclei, applications for online data visualization and real time PC-based algorithms are considered. Special attention is paid to the application of real time techniques for radical suppression of background products in heavy-ion-induced nuclear reactions at the U-400 cyclotron of the Flerov Laboratory of Nuclear Reactions. The detection system of the Dubna gas-filled recoil separator (DGFRS) is also briefly described. Calculated heavy recoil spectra are compared with those measured in heavy-ion-induced nuclear reactions. r 2005 Elsevier B.V. All rights reserved. PACS: 02.70.Lq; 07.05.Tp; 07.81.+a; 29.30.Ag Keywords: Recoil separator; Heavy ion reaction; Silicon radiation detector; Computer simulation
1. Introduction The problem of experimental verification of the existence of the hypothetical domain of super-heavy nuclides is one of the fundamental outcomes of the nuclear shell model that has been extensively discussed in recent years. Another problem is the existence of even stronger spherical shells beyond 208Pb, in the domain of heavier neutron-rich nuclei with Z ¼ 114 (possibly 120, 122 or 126) and N ¼ 184. Since 1998, the Dubna gas-filled recoil separator (DGFRS) heavy-element research group has attempted to verify this non-trivial theoretical hypothesis [1–6]. This region is not easily reached with stable partner nuclei. In order to approach the N ¼ 184 shell, maximum neutron excess is needed in both the target and projectile nuclei. With this aim, we used targets of enriched isotopes such as Pu, Am, Cm and Cf, and the rare and expensive isotope 48Ca as a projectile. We used the main U-400 cyclotron of the Flerov Laboratory of Nuclear Reactions (FLNR) to accelerate 48 Ca ions and the DGFRS to separate in flight the reaction products under investigation from different backgrounds. The separator was filled with hydrogen at a pressure of approximately 1 Torr [7]. Evaporation residue (ER) recoil Corresponding author. Tel.: +7 09621 64246; fax: +7 09621 65083.
E-mail address: [email protected] (Y.S. Tsyganov). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.11.044
passed through a time-of-flight (TOF) system, and was implanted into a 4 12 cm2 semiconductor detector array with 12 vertical position-sensitive strips [8–11]. This detector was surrounded by eight 4 4 cm2 detectors to provide detection efficiency for alpha decay registration of up to 87% of 4p. The full-width at half-maximum (FWHM) for position resolution of the signals from correlated decays of nuclei implanted in the detectors was 0.8–1.3 mm for ER-alpha signals and 0.5–0.8 mm for ERspontaneous fission (SF) signals. The PC-based DGFRS data acquisition system provides not only data storage event by event, allowing the accumulation of more than hundred working histograms, but also the visualization and control of definite parameters related to the detection module and the separator set-up. 2. Background products Although the DGFRS (and its analogues, see Ref. [7]) is a highly effective set-up for ER separation, most of the signals detected at the focal plane of the separator during long-term experiments are of a background nature. This is due to different reasons, such as an extremely low crosssection of the product under investigation (sometimes units of picobarns or lower), very intense beams of heavy ions coming from the cyclotron, transfer reaction products,
ARTICLE IN PRESS 330
Y.S. Tsyganov, A.N. Polyakov / Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332
neutron-induced signals in a passivated implanted silicon (PIPS) detector, etc. [9]. The typical counting rate is approximately tens of events per second in the focal plane detector, depending on the cyclotron tuning and reaction conditions. This means that more than 1.0 108 background signals are measured by the detection system during a 1 month experiment, whereas only single detected events can be attributed to the complete fusion products. A significant proportion of the background particles are charged (alpha decay events being the most significant part of the experimental data flow) and, therefore, can be detected by a TOF and/or a veto detector [9]. Of course, neutron-like backgrounds cannot be suppressed with a TOF detector and in this sense, can simulate signals such as alpha particles in a silicon detector. A typical value for the latter parameter is approximately 0.2 min–1 per strip for signals in the energy interval 9.6–11.0 MeV. 3. Real time detection mode for recoil–alpha-correlated sequences In many experiments we used a special detection mode to detect two or more sequential decays. The beam was switched-off after a recoil signal was detected with parameters of implanted energy and TOF expected for the evaporation residues, followed by an alpha-like signal within preset energy and time intervals in the same strip, within a position window corresponding to the position resolution. Thus, all the expected sequential decays of the daughter nuclides should be observed in the absence of a beam-associated background. We used a more complicated form of beam switching based on recoil–alpha correlation chain real time detection, except for single recoil detection [18] to minimize total experimental efficiency losses for correlation times up to tens of seconds. In a more detailed form, it can easily be shown that the equation relating to these two approaches from the viewpoint of equivalence of total experimental efficiency losses is rffiffiffiffiffi tR tRa ¼ , (1) na where tRa is the recoil–alpha correlation time, tR is the recoil time (duration of pause, generated by recoil detection [18]) and na is the signal rate, such as alpha particles per strip and per actual vertical position element. The real time algorithm for searching recoil–alpha sequences operates in parallel with data capture and file writing. It should be noted that such parallelism is achieved by applying a specially designed autonomous CAMAC crate controller [12] operating together with intermediate buffer memory for the main data flow, whereas an intellectual KK-012 crate controller [13] operates with the imminent event to find a correlated recoil–alpha pair. The basic idea of the algorithm, in brief, is that it uses the discrete representation of the PIPS detector separately for
recoils and alpha particle signals. Thus, the real detector is presented in the form of two matrixes, one for ‘‘ER’’ and the other for ‘‘alpha particles’’ [9,14,15], with elapsed time as a matrix element in both cases, whereas the matrix indexes correspond to strip number and vertical position (in discrete representation). In addition [9], to prolong the ‘‘beam OFF’’ interval up to tens of minutes or hours, information about the strip number is used, as well as extraction of the next (after switch-off) alpha particle with its vertical position, although with a wider position interval in comparison with one generating the beam switch-off. Thus, modified in comparison with Ref. [9], the working system of equations is as follows: ai N a þ bi R0i R0i a;esc;ER j ¼ int N max þ1 þ di , ayi þ byi Ri Ri (2) ¼ tðelapsedÞ, ta;ER i;j
(3)
a1;a2 1;2 ti;jþk tER i;j pT pr ðE a1;2 Þ ðrecoil2alphaÞ,
(4)
(Usually Tpr is a constant, but in the same experiments a relation of the form T 10½ðaZ þ bÞQ21=2 þ cZ þ d is used, where a ¼ 1:78, b ¼ 21:398, c ¼ 0:25488 and d ¼ 28:423. Q is the estimated decay energy.) or: a1 a ta2 i;jþk ti;j pT pr ðE a1;2 Þ ðalpha2alphaÞ,
(5)
a1;2 ta_OFF i;jþ2k ti;j pdT OFF
) dT OFF ¼ KdT OFF ðbeam-OFFprolongationÞ.
ð6Þ
In these equations, T pr ðE a1;2 Þ are the preset time intervals for the first and second (if the first escapes with a position signal below the 500–1000 keV threshold) alpha particles, respectively; N max ¼ 170 is the maximum value of cells per strip in a discrete representation; i ¼ 1; . . . ; 12 is the strip number; k ¼ 3; . . . ; þ3 is the cell index [9,14,15]; K ¼ 10–20 is the prolongation factor; and dTOFF is the duration of the beam-OFF interval after detection of the first correlation chain. Subscript a_OFF indicates that the appropriate alpha particle is detected in the beam-OFF interval. Other parameters in Eqs. (2)–(6) are calibration constants that were extracted from the calibration reactions and parameters of the PIPS detector. The calibration parameter d is equal to zero in the ER case, and is a small, non-zero value in the case of alpha decay in the focal plane detector and for an alpha particle escaping this detector, detected in the backward detector and having a non-zero component in the focal plane detector (superscripts a and esc, respectively). It is extracted from the calibration reactions natYb+48Ca and 207Pb+48Ca. Subscripts 1 and 2 for a1,2 indicate the first and second (forthcoming) alpha particles that cause the beam-OFF pause. Figs. 1(a) and (b) shows two time sequences in the decay chains observed at 48Ca energy values of (a) E L ¼ 248 MeV and (b) E L ¼ 253 MeV, as well as the energy measured,
ARTICLE IN PRESS Y.S. Tsyganov, A.N. Polyakov / Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332
331
300
9.1 MeV
α1 α2 280
α3 276
α4 272
α5
Bh
115
111 18.7 mm
275
α4 271
sd 2.3 simulated
Mt
115
200
10.59 MeV 46.6 ms
113 17.6 mm 10.12 MeV 0.147 s
Mean 11.0 2.1 std
10
100
measured 5
111 17.6 mm 10.37 MeV 0.245 s
0
0 0
10.33 MeV 14.0 ms
(a)
5
10
15
20
0
25
4
(b)
E, MeV
8
12
16
20
E, MeV
Bh 17.7 mm
19.2 mm
α
8.97 MeV 15.388 s
279
α3
9.76 MeV 1.793 s
283
α2
10.00 MeV 0.517 s
9.74 MeV 1.834 s
α1
10.50 MeV 280 ms
113 18.5 mm
Mt 18.1 mm
Mean 11.4
18.0 mm
287
288
284
12.2MeV
R
18.8 mm
counts
R
300
The beam was switched off
18.1 mm
Db
Db
SF
SF 206 MeV 105.96 min
140 MeV 16.80 h
(a)
17.8 mm
counts
267
268
(b)
100
17.9 mm
Fig. 1. Two sequences of the decay chains observed at different energy values: (a) E L ¼ 248 MeV and (b) E L ¼ 253 MeV.
200
48
0
Ca
time intervals and vertical positions, with respect to the top of the strips for the decay events observed [1]. During the last 3 yr, more than 50 decay chains for Z ¼ 110–116 were detected under low-background conditions in long-term experiments. 4. Super-heavy recoil spectra calculation in a silicon radiation detector When applying the above technique, a reasonable question is how to estimate the shape and position of an ER spectrum due to the relatively high pulse-height defect (PHD) in a silicon radiation detector. There are several ways that are actually complementary to each other. Estimation based on the measured mass of the ER is one of these. Of course, of great interest is the possibility of computer simulation based on knowledge of the energy losses and broadening in different media [16], the neutron evaporation contribution to broadening, and PHD and its fluctuations in silicon. The code described in Ref. [16] allows a simulation that takes into account the reasons mentioned for transformation of the spectra originating in the target to that registered by a PIPS detector. Figs. 2(a)–(c) shows calculated and measured spectra of registered energy for the two reactions. Note that Fig. 2c corresponds to the GSI experiment aimed at the synthesis of the Z ¼ 112 element some years ago [17] and that Fig. 2a included no free parameters in the simulation procedure. Special attention should be paid to the fact that the event marked by the left arrow (2c) on the histogram was eliminated, as described in [18], after careful analysis of the raw data. In May 2000 the experiment 70Zn+208Pb112+1n was repeated by the same group [18]. One additional event of element 112 with ER energy of 24.1 MeV was reported. This event is shown in Fig. 2c
15
(c)
20
25
30
35
40
E, MeV
Fig. 2. (a) Simulated and (b) measured (Z ¼ 114–116) spectra of evaporation residue recoils. (c) Simulated spectrum of super heavy recoils for 208Pb+70Zn-112+1n nuclear reaction. Detected events [17,18] are shown by arrows.
with an appropriate comment. The remaining questions are the formation mechanism for PHD, especially its recombination component, and a more exhaustive discussion of the applicability and limits of the overall ‘‘one event–one element’’ philosophy, but such a discussion is outside the scope of this paper.1 5. Conclusions Together with the development and improvement of both accelerating and separation techniques, computer codes applied in active form not only allow data capture and visualization, but also play an even more significant part in improving the overall experimental conditions from the viewpoint of effect/background ratio. This resulted in the possibility of establishing genetic links between alpha particles for decay times of up to tens of hours or even days. We plan to use this approach in forthcoming experiments with an estimated cross-section for complete fusion reaction product below 1 pb. When preparing this manuscript, an experiment [19] on chemical identification of the 268Db isotope as a product terminating decay chains of the Z ¼ 115 element from the reaction 243Am+48Ca-288115+3n was successfully performed. This confirmed the decay properties of 268Db measured previously [1]. 1
In the case of large (tens of percent) recombination components, it should be measured, although for model recoil of approximately Z ¼ 100 with comments on a reasonable type of recombination in silicon.
ARTICLE IN PRESS 332
Y.S. Tsyganov, A.N. Polyakov / Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332
Acknowledgements The authors are indebted to Drs. V.K. Utyonkov, A.M. Sukhov, I.V. Shirokovsky and S. Iliev for their assistance and fruitful discussions on the item reported in this paper. References [1] [2] [3] [4] [5]
Yu.Ts. Oganessian, et al., Phys. Rev. C 69 (2004) 021601(R). Yu.Ts. Oganessian, et al., Phys. Rev. C 69 (2004) 054607. Yu.Ts. Oganessian, et al., Eur. Phys. J. A 15 (2002) 201. Yu.Ts. Oganessian, et al., Nucl. Phys. A 734 (2004) 109. Yu.Ts. Oganessian, V.K. Utyonkov, Yu.V. Lobanov, et al., Rev. Mex. Fis. 46 (Suppl. 1) (2000) 35. [6] Yu.Ts. Oganessian, et al., Phys. Rev. C 64 (2001) 054606. [7] K. Subotic, et al., Nucl. Instr. and Meth. Phys. Res. A 481 (2002) 71. [8] Yu.S. Tsyganov, V.G. Subbotin, A.N. Polyakov, et al., Nucl. Instr. and Meth. Phys. Res. A 392 (1997) 197.
[9] Yu.S. Tsyganov, V.G. Subbotin, A.N. Polyakov, et al., Nucl. Instr. and Meth. Phys. Res. A 525 (2004) 213. [10] Yu. Tsyganov, JINR Commun. P-10-98-20, Dubna, 1998 (in Russian). [11] Yu. Tsyganov, JINR Commun. E-10-99-36, Dubna, 1999. [12] Yu.S. Tsyganov, et al., in: Proceedings of High Performance Computing on the Information Superhighway, HPC Asia-97, 28 April–2 May 1997, Seoul, Korea, IEEE Computer Society Press, Los Alamos, CA, 1997, p. 651. [13] I.N. Churin, et al., JINR Commun. P10-90-589, Dubna, 1990 (in Russian). [14] Yu.S. Tsyganov, A.N. Polyakov, A.M. Sukhov, Nucl. Instr. and Meth. Phys. Res. A 513 (2003) 413. [15] Yu. Tsyganov, J. Phys. G Nucl. Part. Phys. 25 (1999) 937. [16] Yu. Tsyganov, Nucl. Instr. and Meth. Phys. Res. A 378 (1996) 356. [17] S. Hofmann, et al., Zh. Phys. A 354 (1996) 229. [18] S. Hofmann, et al., Eur. Phys. J. A 14 (2002) 147; V. Ninov, K.E. Gregorich, W. Loveland, et al., Phys. Rev. Lett. 89 (3) (2002) 039901(E). [19] Yu.Ts. Oganessian, et al., Phys. Rev. C 72 (2005) 034611.
Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332 www.elsevier.com/locate/nima
Computing at the Dubna gas-filled recoil separator Yuri S. Tsyganov, Alexandr N. Polyakov Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russian Federation Available online 1 December 2005
Abstract Simulation codes for the spectra of heavy implanted nuclei, applications for online data visualization and real time PC-based algorithms are considered. Special attention is paid to the application of real time techniques for radical suppression of background products in heavy-ion-induced nuclear reactions at the U-400 cyclotron of the Flerov Laboratory of Nuclear Reactions. The detection system of the Dubna gas-filled recoil separator (DGFRS) is also briefly described. Calculated heavy recoil spectra are compared with those measured in heavy-ion-induced nuclear reactions. r 2005 Elsevier B.V. All rights reserved. PACS: 02.70.Lq; 07.05.Tp; 07.81.+a; 29.30.Ag Keywords: Recoil separator; Heavy ion reaction; Silicon radiation detector; Computer simulation
1. Introduction The problem of experimental verification of the existence of the hypothetical domain of super-heavy nuclides is one of the fundamental outcomes of the nuclear shell model that has been extensively discussed in recent years. Another problem is the existence of even stronger spherical shells beyond 208Pb, in the domain of heavier neutron-rich nuclei with Z ¼ 114 (possibly 120, 122 or 126) and N ¼ 184. Since 1998, the Dubna gas-filled recoil separator (DGFRS) heavy-element research group has attempted to verify this non-trivial theoretical hypothesis [1–6]. This region is not easily reached with stable partner nuclei. In order to approach the N ¼ 184 shell, maximum neutron excess is needed in both the target and projectile nuclei. With this aim, we used targets of enriched isotopes such as Pu, Am, Cm and Cf, and the rare and expensive isotope 48Ca as a projectile. We used the main U-400 cyclotron of the Flerov Laboratory of Nuclear Reactions (FLNR) to accelerate 48 Ca ions and the DGFRS to separate in flight the reaction products under investigation from different backgrounds. The separator was filled with hydrogen at a pressure of approximately 1 Torr [7]. Evaporation residue (ER) recoil Corresponding author. Tel.: +7 09621 64246; fax: +7 09621 65083.
E-mail address: [email protected] (Y.S. Tsyganov). 0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.11.044
passed through a time-of-flight (TOF) system, and was implanted into a 4 12 cm2 semiconductor detector array with 12 vertical position-sensitive strips [8–11]. This detector was surrounded by eight 4 4 cm2 detectors to provide detection efficiency for alpha decay registration of up to 87% of 4p. The full-width at half-maximum (FWHM) for position resolution of the signals from correlated decays of nuclei implanted in the detectors was 0.8–1.3 mm for ER-alpha signals and 0.5–0.8 mm for ERspontaneous fission (SF) signals. The PC-based DGFRS data acquisition system provides not only data storage event by event, allowing the accumulation of more than hundred working histograms, but also the visualization and control of definite parameters related to the detection module and the separator set-up. 2. Background products Although the DGFRS (and its analogues, see Ref. [7]) is a highly effective set-up for ER separation, most of the signals detected at the focal plane of the separator during long-term experiments are of a background nature. This is due to different reasons, such as an extremely low crosssection of the product under investigation (sometimes units of picobarns or lower), very intense beams of heavy ions coming from the cyclotron, transfer reaction products,
ARTICLE IN PRESS 330
Y.S. Tsyganov, A.N. Polyakov / Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332
neutron-induced signals in a passivated implanted silicon (PIPS) detector, etc. [9]. The typical counting rate is approximately tens of events per second in the focal plane detector, depending on the cyclotron tuning and reaction conditions. This means that more than 1.0 108 background signals are measured by the detection system during a 1 month experiment, whereas only single detected events can be attributed to the complete fusion products. A significant proportion of the background particles are charged (alpha decay events being the most significant part of the experimental data flow) and, therefore, can be detected by a TOF and/or a veto detector [9]. Of course, neutron-like backgrounds cannot be suppressed with a TOF detector and in this sense, can simulate signals such as alpha particles in a silicon detector. A typical value for the latter parameter is approximately 0.2 min–1 per strip for signals in the energy interval 9.6–11.0 MeV. 3. Real time detection mode for recoil–alpha-correlated sequences In many experiments we used a special detection mode to detect two or more sequential decays. The beam was switched-off after a recoil signal was detected with parameters of implanted energy and TOF expected for the evaporation residues, followed by an alpha-like signal within preset energy and time intervals in the same strip, within a position window corresponding to the position resolution. Thus, all the expected sequential decays of the daughter nuclides should be observed in the absence of a beam-associated background. We used a more complicated form of beam switching based on recoil–alpha correlation chain real time detection, except for single recoil detection [18] to minimize total experimental efficiency losses for correlation times up to tens of seconds. In a more detailed form, it can easily be shown that the equation relating to these two approaches from the viewpoint of equivalence of total experimental efficiency losses is rffiffiffiffiffi tR tRa ¼ , (1) na where tRa is the recoil–alpha correlation time, tR is the recoil time (duration of pause, generated by recoil detection [18]) and na is the signal rate, such as alpha particles per strip and per actual vertical position element. The real time algorithm for searching recoil–alpha sequences operates in parallel with data capture and file writing. It should be noted that such parallelism is achieved by applying a specially designed autonomous CAMAC crate controller [12] operating together with intermediate buffer memory for the main data flow, whereas an intellectual KK-012 crate controller [13] operates with the imminent event to find a correlated recoil–alpha pair. The basic idea of the algorithm, in brief, is that it uses the discrete representation of the PIPS detector separately for
recoils and alpha particle signals. Thus, the real detector is presented in the form of two matrixes, one for ‘‘ER’’ and the other for ‘‘alpha particles’’ [9,14,15], with elapsed time as a matrix element in both cases, whereas the matrix indexes correspond to strip number and vertical position (in discrete representation). In addition [9], to prolong the ‘‘beam OFF’’ interval up to tens of minutes or hours, information about the strip number is used, as well as extraction of the next (after switch-off) alpha particle with its vertical position, although with a wider position interval in comparison with one generating the beam switch-off. Thus, modified in comparison with Ref. [9], the working system of equations is as follows: ai N a þ bi R0i R0i a;esc;ER j ¼ int N max þ1 þ di , ayi þ byi Ri Ri (2) ¼ tðelapsedÞ, ta;ER i;j
(3)
a1;a2 1;2 ti;jþk tER i;j pT pr ðE a1;2 Þ ðrecoil2alphaÞ,
(4)
(Usually Tpr is a constant, but in the same experiments a relation of the form T 10½ðaZ þ bÞQ21=2 þ cZ þ d is used, where a ¼ 1:78, b ¼ 21:398, c ¼ 0:25488 and d ¼ 28:423. Q is the estimated decay energy.) or: a1 a ta2 i;jþk ti;j pT pr ðE a1;2 Þ ðalpha2alphaÞ,
(5)
a1;2 ta_OFF i;jþ2k ti;j pdT OFF
) dT OFF ¼ KdT OFF ðbeam-OFFprolongationÞ.
ð6Þ
In these equations, T pr ðE a1;2 Þ are the preset time intervals for the first and second (if the first escapes with a position signal below the 500–1000 keV threshold) alpha particles, respectively; N max ¼ 170 is the maximum value of cells per strip in a discrete representation; i ¼ 1; . . . ; 12 is the strip number; k ¼ 3; . . . ; þ3 is the cell index [9,14,15]; K ¼ 10–20 is the prolongation factor; and dTOFF is the duration of the beam-OFF interval after detection of the first correlation chain. Subscript a_OFF indicates that the appropriate alpha particle is detected in the beam-OFF interval. Other parameters in Eqs. (2)–(6) are calibration constants that were extracted from the calibration reactions and parameters of the PIPS detector. The calibration parameter d is equal to zero in the ER case, and is a small, non-zero value in the case of alpha decay in the focal plane detector and for an alpha particle escaping this detector, detected in the backward detector and having a non-zero component in the focal plane detector (superscripts a and esc, respectively). It is extracted from the calibration reactions natYb+48Ca and 207Pb+48Ca. Subscripts 1 and 2 for a1,2 indicate the first and second (forthcoming) alpha particles that cause the beam-OFF pause. Figs. 1(a) and (b) shows two time sequences in the decay chains observed at 48Ca energy values of (a) E L ¼ 248 MeV and (b) E L ¼ 253 MeV, as well as the energy measured,
ARTICLE IN PRESS Y.S. Tsyganov, A.N. Polyakov / Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332
331
300
9.1 MeV
α1 α2 280
α3 276
α4 272
α5
Bh
115
111 18.7 mm
275
α4 271
sd 2.3 simulated
Mt
115
200
10.59 MeV 46.6 ms
113 17.6 mm 10.12 MeV 0.147 s
Mean 11.0 2.1 std
10
100
measured 5
111 17.6 mm 10.37 MeV 0.245 s
0
0 0
10.33 MeV 14.0 ms
(a)
5
10
15
20
0
25
4
(b)
E, MeV
8
12
16
20
E, MeV
Bh 17.7 mm
19.2 mm
α
8.97 MeV 15.388 s
279
α3
9.76 MeV 1.793 s
283
α2
10.00 MeV 0.517 s
9.74 MeV 1.834 s
α1
10.50 MeV 280 ms
113 18.5 mm
Mt 18.1 mm
Mean 11.4
18.0 mm
287
288
284
12.2MeV
R
18.8 mm
counts
R
300
The beam was switched off
18.1 mm
Db
Db
SF
SF 206 MeV 105.96 min
140 MeV 16.80 h
(a)
17.8 mm
counts
267
268
(b)
100
17.9 mm
Fig. 1. Two sequences of the decay chains observed at different energy values: (a) E L ¼ 248 MeV and (b) E L ¼ 253 MeV.
200
48
0
Ca
time intervals and vertical positions, with respect to the top of the strips for the decay events observed [1]. During the last 3 yr, more than 50 decay chains for Z ¼ 110–116 were detected under low-background conditions in long-term experiments. 4. Super-heavy recoil spectra calculation in a silicon radiation detector When applying the above technique, a reasonable question is how to estimate the shape and position of an ER spectrum due to the relatively high pulse-height defect (PHD) in a silicon radiation detector. There are several ways that are actually complementary to each other. Estimation based on the measured mass of the ER is one of these. Of course, of great interest is the possibility of computer simulation based on knowledge of the energy losses and broadening in different media [16], the neutron evaporation contribution to broadening, and PHD and its fluctuations in silicon. The code described in Ref. [16] allows a simulation that takes into account the reasons mentioned for transformation of the spectra originating in the target to that registered by a PIPS detector. Figs. 2(a)–(c) shows calculated and measured spectra of registered energy for the two reactions. Note that Fig. 2c corresponds to the GSI experiment aimed at the synthesis of the Z ¼ 112 element some years ago [17] and that Fig. 2a included no free parameters in the simulation procedure. Special attention should be paid to the fact that the event marked by the left arrow (2c) on the histogram was eliminated, as described in [18], after careful analysis of the raw data. In May 2000 the experiment 70Zn+208Pb112+1n was repeated by the same group [18]. One additional event of element 112 with ER energy of 24.1 MeV was reported. This event is shown in Fig. 2c
15
(c)
20
25
30
35
40
E, MeV
Fig. 2. (a) Simulated and (b) measured (Z ¼ 114–116) spectra of evaporation residue recoils. (c) Simulated spectrum of super heavy recoils for 208Pb+70Zn-112+1n nuclear reaction. Detected events [17,18] are shown by arrows.
with an appropriate comment. The remaining questions are the formation mechanism for PHD, especially its recombination component, and a more exhaustive discussion of the applicability and limits of the overall ‘‘one event–one element’’ philosophy, but such a discussion is outside the scope of this paper.1 5. Conclusions Together with the development and improvement of both accelerating and separation techniques, computer codes applied in active form not only allow data capture and visualization, but also play an even more significant part in improving the overall experimental conditions from the viewpoint of effect/background ratio. This resulted in the possibility of establishing genetic links between alpha particles for decay times of up to tens of hours or even days. We plan to use this approach in forthcoming experiments with an estimated cross-section for complete fusion reaction product below 1 pb. When preparing this manuscript, an experiment [19] on chemical identification of the 268Db isotope as a product terminating decay chains of the Z ¼ 115 element from the reaction 243Am+48Ca-288115+3n was successfully performed. This confirmed the decay properties of 268Db measured previously [1]. 1
In the case of large (tens of percent) recombination components, it should be measured, although for model recoil of approximately Z ¼ 100 with comments on a reasonable type of recombination in silicon.
ARTICLE IN PRESS 332
Y.S. Tsyganov, A.N. Polyakov / Nuclear Instruments and Methods in Physics Research A 558 (2006) 329–332
Acknowledgements The authors are indebted to Drs. V.K. Utyonkov, A.M. Sukhov, I.V. Shirokovsky and S. Iliev for their assistance and fruitful discussions on the item reported in this paper. References [1] [2] [3] [4] [5]
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