4π Neutron detection with low-intensity radioactive beams

ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 581 (2007) 783–790 www.elsevier.com/locate/nima

4p Neutron detection with low-intensity radioactive beams A. Del Zoppoa,, P. Figueraa, A. Musumarraa,b, N. Colonnad, R. Albaa, C. Bonomoa,1, S. Cherubinia,b, L. Cosentinoa, A. Di Pietroa, M. Gulinoa,b, M. La Cognataa, L. Lamiaa,b, M.G. Pellegritia,2, R.G. Pizzonea, C. Rolfsc, S. Romanoa,b, C. Spitaleria,b, S. Tudiscoa, A. Tuminoa,b a

INFN-Laboratori Nazionali del Sud, Via S.Sofia 62, I95123 Catania, Italy Dipartimento di Metodologie Fisiche e Chimiche per l’Ingegneria, Universita` di Catania, I95123 Catania, Italy c Institut fur Physik mit Ionenstrahlen, Ruhr-Universitaet Bochum, Bochum, Germany d INFN-Sezione di Bari, Via Orabona 4, I70126, Bari, Italy

b

Received 6 July 2007; received in revised form 25 July 2007; accepted 1 August 2007 Available online 12 August 2007

Abstract The feasibility of inclusive neutron production measurements in reactions induced by low-intensity radioactive beams using a 4p thermalization counter is studied. The time response of the detector is investigated experimentally by a technique that results in an enhanced sensitivity to weak components with long capture times. Complementary Monte Carlo simulations are presented. The capture time response is found to be independent on the neutron energy above 0.1 MeV. The capability of the capture time information in the unambiguous identification of neutron signals correlated to the projectile arrival on the target even in the presence of an intense background contamination is shown. As an application case, the 8Li(4He,n)11B reaction at the Big-Bang temperature is commented. r 2007 Elsevier B.V. All rights reserved. PACS: 25.60.t; 26.35.+c; 29.40.n Keywords: Thermalization detectors; Radioactive ion beams; Big-Bang nucleosynthesis

1. Introduction The recent efforts in radioactive ion beam (RIB) production (e.g. Refs. [1–16]) have enlarged noticeably the field of the experimental exploration and have already lead to the discovery of new nuclear physics phenomena (e.g. Refs. [17–20]). Presently, the typical intensity of weakly exotic RIBs in proximity of the stability valley is in the range 106–108 ions/s. On the other extreme, highly exotic beams, namely nuclei close to the neutron or proton drip lines, are produced with much smaller intensity, of the order of a few ions/s. This work is devoted to those experiments with RIBs where the request for a large beam Corresponding author. Tel.: +39095542280; fax: +390957141815.

E-mail address: [email protected] (A. Del Zoppo). Present address: INFN-Sezione di Ferrara, Italy. 2 Present address: CRC- Louvain la Neuve, Belgium. 1

0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.08.069

intensity is not mandatory. For instance, beam intensities of the order of 104 ions/s or smaller have been shown to be sufficient to reach the desired goals in several nuclear reaction experiments (e.g. Refs. [21,22]). This could be an expanding research field also connected to the availability of low-intensity secondary RIBs using simple production techniques (e.g. Ref. [23]). The kind of measurements considered in this work is characterized by extremely low counting rates. Consequently, high-efficiency detection systems are necessary. In particular, interest may exist in neutron production measurements in nuclear reactions. In this specific context, an attractive solution could be represented by detectors based on neutron thermalization. This type of detector is widely used in applications concerning safeguard (e.g. Ref. [24]), geophysics (e.g. Ref. [25]), fundamental physics (e.g. Ref. [26]), nuclear physics (e.g. Refs. [22,27,28]), etc. By an appropriate choice of the moderator-absorber size in a 4p configuration, a

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large-detection efficiency over a wide energy range is easily achievable. Thus, in principle, thermalization detectors could also allow one to study rare events such as those expected in experiments at RIB facilities. Actually, problems could arise with the background contamination. These can be hardly overcome by simply performing coincidence measurements with other devices, where possible. In fact, the long mean capture time (of the order of 100 ms typically) imposes such a large coincidence time window that the rate of accepted background events, though low on an absolute scale, could be unacceptable. In such a case the only possibility to identify the weak physical signal and to subtract unambiguously the relatively intense background is to characterize the capture time distribution of the neutrons inside the detector. This has been done in this work. Moreover, we have explored the possibility of using the time correlation between the incoming projectile ion and the neutron capture signal. At the low projectile rates considered here (o103 s1) there is, in practice, no constraint on the capture time range of the detector. As a specific example, the cross-section measurement of the 8Li(a,n)11B reaction, a process which possibly allowed overcoming the A ¼ 8 mass gap in Big-Bang nucleosynthesis [29] as well as in the r-process nucleosynthesis [30], is presented. The study presented here has been performed using the Polycube 4p thermalization detector [28,31] presently installed at the Laboratory Nazionali del Sud accelerator facility in Catania. The paper is organized as follows: in Section 2 the Polycube detector is briefly described. In Section 3 the capture time response function of the Polycube, measured with a 252Cf source, is analysed. In Section 4, accurate Monte Carlo simulations of the apparatus are compared with the experimental results. Based on these simulations, a detailed discussion on the behaviour of the device is presented. Finally, an example of application of the detector to a low count rate reaction study is reported in Section 5. 2. The Polycube neutron detector A thermalization detector consists of two parts: the moderating and weakly absorbing medium, where fast neutrons are slowed down to thermal energies, and the strongly absorbing material in which mainly thermal neutron capture occurs. The moderator–absorber Polycube configuration has been presented in detail elsewhere [28,31]. The detector is shown in Fig. 1. Briefly, 12 cylindrical proportional counters (radius ¼ 1.27 cm and length ¼ 50 cm) filled with 3He at the pressure of 4 atm play the role of thermal neutron absorber through the 3 He(n,p)3H reaction, characterized by the Q-value Q ¼ 0.764 MeV and a capture cross-section of 5300 b. The 12 counters are embedded into a 40 cm  40 cm  40 cm polyethylene moderator. The moderator is surrounded by a 0.6 mm thick cadmium shielding and by a 4p passive layer of polyethylene. The 12 3He counters are

Fig. 1. A picture of the Polycube neutron detector.

located parallel to a 11 cm  13 cm  40 cm channel through the Polycube centre that allows for the insertion of the beam pipe and the reaction chamber. With its 4p geometry the detector is well suited to determine the neutron yield integrated over the angular distribution with a total efficiency of 0.23 for isotropically emitted 1 MeV neutrons. The Polycube neutron capture time tcapt distribution is of primary interest for the aim of this work. Except for the initial slowing down time tsd of the order a few microseconds, the capture time mostly reflects the migration time through the polyethylene moderator before the capture in 3He occurs. Moreover, polyethylene is a weak absorber and in a pure moderator configuration the number of neutrons, which survive the capture at the time tXtsd is expected to be an exponential decreasing function of tcapt [32]. In the case of a pure polyethylene moderator having a size as large as that of the Polycube, the neutron mean life is t ¼ l1E/SvS1 ¼ 160 ms [33], /SvS being the mean absorption rate. Because of the large capture cross-section the migration time in 3He is much shorter. Consequently, the presence of 3He in the Polycube shortens the neutron mean life as compared with that in the pure polyethylene alone. In addition, the non-uniformity of the detector configuration is expected to cause the neutron capture (nc) time probability density dPnc/dtcapt to be the superposition of a number of exponential components, namely Xq dPnc k ¼  etcapt =tk dtcapt t k k

(1)

ARTICLE IN PRESS A. Del Zoppo et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 783–790

the index k (X1) running over each decay component characterized by the lifetime tk and the intensity qk (Skqk ¼ 1). In a preliminary unpublished measurement [34] the Polycube capture time distribution was fitted by using a single exponentially decreasing function in Eq. (1) with t ¼ /tcaptSE87 ms. Indeed, we have checked that the measurement [34] does not exclude the existence of additional weak exponentially decreasing components. In the next section we propose to study the inter-time correlations between two neutrons from a 252Cf radioactive source as a powerful technique to disentangle these additional exponential components.

785

105

252Cf

d(t2 − t1)

dNexp

(s−1)

104

103

102

3. Neutron pair capture inter-time distribution measurement using 252Cf The distribution of the inter-times t2t1 occurring between the first and second captured neutrons out of the multiplicity which accompanies the spontaneous fission of 252 Cf was measured at the Laboratori Nazionali del Sud at Catania. In order to optimize the real to spurious coincidence ratio a low-activity (5 fission events/s) 252Cf source was placed at the centre of the Polycube detector. Each 3He counter was connected to a preamplifier and its output was split into two for analogic and logic signal processing. The time interval between the first and the second captured neutrons was measured using a time to amplitude converter (TAC) operating in the 2 ms range, coupled to a suitable electronic chain. The analogic signal produced by each of the 12 counters was also processed and its pulse height was recorded. In the off-line analysis, cuts on the 3He counter pulse height spectra were applied. A lower cut was placed right at the lower limit of the pulse height spectrum, at 1/4 of the 3 He(n,p)3H reaction Q-value. Such a lower limit is obtained when the reaction occurs near the counter wall and the proton deposits all of its energy (3/4Q) in the wall. The upper cut was applied right at the end of the peak corresponding to the full Q-value deposition in the 3He. Then the data were sorted according to the number of detected neutrons ndet, with ndetX2. Finally, the experimental distribution dNexp/d(t2t1) of the inter-times between captured neutrons in events with ndet ¼ 2 was constructed. These events represent as much as 80% of the total number of events with ndetX2. The resulting intertime spectrum is shown in Fig. 2. The distribution dNexp/d(t2t1) was analysed using a formalism based on the assumption that the decay times tk and intensities qk in Eq. (1) are constant in the considered neutron energy range. In the formalism, we considered that in 252Cf nuclear fission events the energy spectrum of the emitted neutrons (en) dnen/dEn exhibits a pure Maxwellian shape, with temperature T ¼ 1.41 MeV [35]. We also checked that the frequencies f nen [24] of 252Cf fission events with multiplicity of emitted neutrons nen can be satisfactorily described by the binomial

10 0

500

1000 t2 − t1 (s)

1500

Fig. 2. Experimental inter-time distribution of neutrons from 252Cf detected in the polycube (histogram) and data fit by Eq. (5) performed with a single (dashed) and a double (solid) exponential form of both the probabilities (1) and (4). The dot-dashed line depicts the background level in the measurement.

Table 1 Experimental normalized frequency f nen of the number of neutrons accompanying the spontaneous fission of 252 Cf (from [24] ) and its analytical description Bnnmax ðhnen i=nmax Þ according to the formula (2) . en 

nen

f nen

Bnnmax en

0 1 2 3 4 5 6 7

0.002 0.024 0.123 0.271 0.306 0.188 0.066 0.016

0.004 0.036 0.127 0.248 0.289 0.203 0.079 0.013

hnen i nmax



The largest relative deviations appear for the small values of the nen ¼ 0 and nen ¼ 1 frequencies. Their effects on the two neutron inter-time formalism developed in section 3 are negligible.

distribution f nen ¼ Bnnmax en



hnen i nmax

 ¼

nmax nen

!    hnen i nen hnen i Nnen 1 nmax nmax (2)

valid for nmax independent events with elementary probability of success equal to hnen i=n  max , with  nmax ¼ 7. In Table 1 the values of Bnnmax hn i=n calculated en max en in this work using (2) are compared with those of f nen in Ref. [24].

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Each of the nen independently emitted neutrons is detected by the Polycube with the effective efficiency R1 ðdnen =dE n ÞOðE n Þ dE n . O ¼ 0 R1 0 ðdnen =dE n Þ dE n The measured value of the efficiency for the 252Cf source is O ¼ 0.21(70.01). Thus, for a given multiplicity of emitted neutrons nen, the multiplicity of detected neutrons ndet ¼ 0,1,2,y,nen ! follows the binomial distribution nen ðOÞ ¼ Bnnen Ondet ð1  OÞnen ndet . Accordingly, by det ndet folding Bnnen ðOÞ with the distribution of emitted neutrons det ðhnen i=nmax Þ, and using the 1nen discussed before, Bnnmax en neutron capture time distribution (1) with decay times tk and intensities qk independent of the neutron energy, the probability of observing the inter-time t2t1 in the class of events with ndet ¼ 2 is expected to be of the form   X q0 nmax dP2nc k ðt2 t1 Þ=tk ¼ e O2 ð1  OÞðnmax 2Þ dðt2  t1 Þ t 2 k k

with mean life tslow ¼ 16970.7 ms and with relative intensities qfast ¼ 0.5870.003 and qslow ¼ 0.4270.003, respectively. Namely, the Polycube neutron capture probability density (1) versus tcapt can be written in the simplified form dPnc qfast t=tfast qslow t=tslow ¼ e þ e dt tfast tslow

(6)

with t ¼ tcapt+tsdEtcapt. Two factors contribute to the unambiguous identification of the slow component: (1) the extremely low level of spurious coincidences in the present measurement; (2) the enhanced sensitivity to weak components with long decay times of the 2-neutron inter-time measurement, compared with single neutron capture time measurement. In fact, in the 2-component case pointed out here, the slow to fast component ratio of the intensities in Eq. (4) is  q0slow qslow qfast ðtslow  tfast Þ þ tfast þ tslow q ¼ 4 slow q0fast qfast qfast ðtslow  tfast Þ þ 2  tfast qfast

strictly valid for n–n inter-time measurements, with the characteristics of the neutron multiplicity and energy distributions of the source as given above. It should be noted that the mathematical form of Eq. (4) versus t2t1 resembles that of the 1-neutron capture probability density (1) versus tcapt. Hence, the 2-neutron inter-time correlation method is sensitive to the set of parameters qk’s and tk’s in Eq.(1). In particular, in the cases where only 1 exponential component exists the 1-neutron capture measurement based on Eq. (1) and the 2-neutron inter-time measurement based on Eq. (4) have the same sensitivity. In all other cases, the 2-neutron inter-time measurement results in an enhanced sensitivity to the exponential components with weaker intensities and longer decay times (see also below). In the analysis, the ndet ¼ 2 coincidence data in Fig. 2 were fitted using the following relation:

namely it is greater than the corresponding ratio of the intensities in Eq. (6). For the Polycube, using the time constant and intensity values determined above, ðq0slow =q0fast Þ  1:6ðqslow =qfast Þ. Qualitatively, the origin of the 2 exponential components is likely related to the presence of 2 weakly absorbing media, polyethylene and air (or vacuum) in the central channel, which mainly determine the Polycube nonuniformity and, consequently, different migration paths and lives of the thermalized neutrons before being captured in 3He. In the above formalism, we assumed the decay times tk and intensities qk in Eq. (1) to be independent on the neutron energy. The comparison with the data in Fig. 2 indicates the validity of this assumption in the range covered by the 252Cf Maxwellian neutron source. To corroborate the possible more general validity of such hypothesis it is instructive to explore Monte Carlo simulations of the Polycube capture time response to mono-energetic neutrons.

dN exp dP2nc ¼A þB dðt2  t1 Þ dðt2  t1 Þ

4. Monte Carlo simulations of the Polycube time response

with q0k ¼ 2qk

P

(4) qh tk =ðtk þ th Þ. We remind that Eq. (4) is

h

(5)

where the scaling constant A and the spurious coincidence rate B were treated as free parameters together with the intensities qk’s and the decay constant tk’s. Results produced by varying the number of components in Eq.(4) are shown in Fig. 2. It is evident that a description based on a single exponential component is inadequate to reproduce the experimental data trend in the whole t2t1 range, where the reduced w2 is w2r  36. The single exponential component seems to fit reasonably the 0–300 ms inter-time range. However, the w2r amounts to approximately 85. The data fit improves noticeably in the whole t2t1 range, with the w2r  1:4, by considering in Eq. (1) 2 exponential components characterized by a fast decay with mean life tfast ¼ 6870.4 ms and by a slow decay

Extensive simulations of the detector were performed with the software tool GEANT 3.21. This package was chosen for the versatility of the geometric and material routines, as well as for the possibility of easily tracking the particles inside the detector. The code has been extensively used for neutron transport in Ref. [36], where the routine MICAP, which is based on point-wise cross-sections from thermal up to 20 MeV neutron energy, was chosen, and it has been proved to give similar results to MCNP [37]. Given the size of the Polycube, the present code does not include molecular cross-sections for any material. Such cross-sections, in fact, directly affect only the buckling term, which in this case is negligible.

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20

En=1 MeV

10 Y (cm)

104

0 −10

103

Counts

−20 −20

102

0 X (cm)

20

10

1 0

200

400

600 800 1000 Capture time (s)

1200

1400

20 4

10

En=25 meV Y (cm)

10

0 −10

103

−20

Counts

A software replica of the Polycube, which included all geometric and material details of the apparatus, was implemented in the Monte Carlo simulation. Neutrons were generated isotropically in the centre of the detector. For each neutron captured in any of the material composing the detector (polyethylene, 3He, Cd, etc.), the capture time tcapt between emission and capture was recorded, together with the position and the material in which it occurred. First of all we have verified the ability of the simulation code to correctly reproduce the experimental 2-neutron inter-time data, measured with the 252Cf source, already shown in Fig. 2. For the 252Cf source, neutrons were simulated according to the known Maxwellian energy distribution [35] and multiplicity distribution f nen [24] already adopted in Section 3. Like in the measurement, only neutron capture events in 3He were considered for the simulation of the detector output. For the specific case of the 252Cf source, the simulations were repeated with the code MCNP, which uses the molecular cross-sections for polyethylene. A comparison between the results of the 2 Monte Carlo simulations provided confidence on the reliability of the GEANT results. The comparison with the experimental data is reported in Fig. 3 where a striking agreement is observed. At this point a systematic campaign was undertaken consisting of a number of simulations performed with mono-energetic neutrons at several incident energies from thermal to 20 MeV. As done for the experimental data in Section 3, the simulated distributions were fitted by the superposition of exponential functions (6) with decay times t’s and intensities q’s treated as free parameters. We found that, in agreement with the two-neutron capture inter-time data

787

−20

102

0 X (cm)

20

10

105

1 104

252Cf

d(t2 −t1)

dNexp

(s−1)

GEANT simulation Experimental data

0

200

400

600 800 1000 Capture time (s)

1200

1400

Fig. 4. GEANT 3.21 simulations (histograms) of the capture time distribution for 1 MeV neutrons (a) and for thermal neutrons (b). The continuous curves represent their best fits according to Eq. (6). The dotted and dashed lines in Fig. 4(a) show the 2 exponential components. The insets show the projections of the capture positions on a plane transverse to the detector central channel.

103

102

10 0

500

1000 t2 − t1 (s)

1500

Fig. 3. The results of the Monte Carlo simulations compared with the experimental inter-time distribution already shown in Fig. 2.

analysis of Fig. 2, the 2-component description (6) is well suited to describe the simulated capture time distributions at neutron energies greater than 100 eV. An example is shown in Fig. 4a for 1 MeV neutrons. On the other hand, at very low energy, the 2-exponential description is not valid any more. In fact, with decreasing neutron energy, the spectrum is progressively shifted towards higher capture time values and, after an initial rise, it exhibits a single

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5. Reaction neutron measurement with a low-intensity radioactive beam

slow

150

100 fast

50

1

0.75

0.5

0.25

0

Altogether the measurement in Section 3 and the simulations in Section 4 show a trend of the Polycube capture time probability density (6) independent on the neutron energy, above Enffi0.1 MeV. Among the 4 parameters tfast, tslow, qfast and qslow ¼ 1qfast which enter into (6) the largest deviation from a constant behaviour is exhibited by tslow which varies of 712% (edge to edge) and of 77% (variance square root) in the neutron energy range 0.1–20 MeV. Such an energy range is of noticeable interest in a great variety of applications. Thus, in general, if a suitable time zero reference correlated with the neutron production time is available, then the experimental capture time spectrum is expected of the form dNðtcapt Þ dPnc ðtcapt Þ ¼a þb dtcapt dtcapt

200

Time Constants (s)

slowly decreasing exponential component. In the limit case of thermal neutrons shown in Fig. 4b, the onset of the decreasing exponential-like trend occurs at tcaptE200 ms. As indicated by the spatial density of the capture events reported in the inset of Fig. 4b, most of the initially thermally emitted neutrons migrate in the polyethylene in proximity to the channel walls, with possible single or multiple traversing of the central empty channel. This prolongs the life of the neutrons, also of those captured in 3 He. As shown by the solid curve in Fig. 4b, a satisfactory description of the rise and fall behaviour of the thermally emitted neutron capture time distribution can be obtained using a function of the type ð1  eðtcapt tshift Þ=trise Þ eðtcapt tshift Þ=tslow for tcaptXtshift, with trise ¼ 10272 ms, tslow ¼ 17671 ms. The parameter tshift ¼ 5272 ms corresponds to the mean time needed to the initially thermal neutron to cover the source–polyethylene distance in the empty channel. The results of the fits of all the simulated capture time curves are summarized in Fig. 5. Here the dependence of the parameters tfast, tslow and qslow in Eq.(6) on the initial neutron energy, from thermal to 20 MeV, is shown. At the 252 Cf mean neutron energy the simulations are in quite good agreement with the corresponding experimental value determined in Section 3 also reported in Fig. 5. Another relevant result is that for 0.1oEno20 MeV the Monte Carlo simulations support the hypothesis made in Section 3 that the parameters tfast, tslow, qfast and qslow can be considered reasonably constant. It should also be noted that the slow exponential component contributes to at least 40% of the total number of neutron capture events.

qslow

788

(7)

with dPnc/dtcapt given by Eq. (6) and the factor b accounting for the uncorrelated coincidence background. We show in this section that, despite the large detection time scale, the precise knowledge of the Polycube time response Eq. (6) allows the identification of the neutron

10−1

10

103 En (eV)

105

107

Fig. 5. Dependence of tfast, tslow (a) and qslow (b) on the initial neutron energy as obtained by the Monte Carlo simulations. For neutron energy below 1 eV the parameters have been obtained by a single exponential fit in the tcapt4200 ms range. The stars represent the values obtained in the analysis of the 252Cf neutron inter-time distribution data in Fig. 2.

correlated signal even in presence of a severe background contamination. The example reported in the following is a nuclear physics application in which the capture time information in a purely inclusive neutron counting procedure allows for the determination of the cross-section from the measurement of the factor a in Eq. (7). For simplicity, we have focused our study to processes in which the 1-n is the only open neutron production exit channel. This is the case of the 8Li(4He,n)11B reaction at centre of mass energies below 2 MeV. This reaction is of great importance for the possible production of ‘‘metals’’ (C,N,O, etc.) during the primordial nucleosynthesis in the inhomogeneous Big-Bang model [29]. In this

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cosmological scenario the Gamow region is in the centre of mass energy range 0.6–0.8 MeV. Details of the experimental procedures are reported in Ref. [22]. Briefly, a radioactive 8Li secondary beam (t1/2 ¼ 840 ms) was produced via the inverse kinematics 7Li+2H-8Li+1H reaction, separated in flight [23] and transported into an 4He gas target located at the centre of the Polycube. The centre of mass energy spanned inside the gas target was 1.2570.2 MeV, close to the BigBang energy range. The laboratory neutron energy was in the 0.1–10 MeV range. The production time of each captured neutron was established by monitoring the projectile arrival time onto the target. To minimize the distortions on the capture spectrum caused by the stochastic nature of the projectile arrival inter-time, the measurement was performed at the mean rate of 100 projectiles/s. Fig. 6 shows the experimental capture time spectrum measured in Ref. [22] versus the 2 exponential component probability distribution (6) determined in the present work. The count statistics reflects the short time of the measurement (70 h) but, nevertheless, the data show clearly the linear dependence (7) expected for correlated neutron production. The slope of the linear best fit leads to the 8Li(4He,n)11B reaction crosssection value of 530 mb with an uncertainty of 180 mb that is comparable with that typically reported in the literature [38]. It should be noted that by adopting the single exponential description characterized by t ¼ 114 ms, as deduced from the 1-component fit of the data in Fig. 2, one would systematically overestimate the cross-section by as much as 20%. In the example discussed here, the reaction neutron rate is  5  104 neutrons/s and the background rate, mainly caused by the primary 7Li beam separation procedure adopted in Ref. [23], is  2  101 neutrons/s with a 1:400 S/N ratio. Therefore, in a purely inclusive counting measurement the background would have completely obscured the reaction

tcapt (s) 200

100

50

20

8

ΔN / Δtcapt (s−1)

8Li (α,n)11B

Ec.m. =1.25±0.20 MeV

7.5

7

789

neutron signal. After implementing the analysis technique based on the capture time information as in (7) and Fig. 6, the use of the projectile-neutron coincidence in a time window of the order of a few tslow makes the S/N ratio to increase up to a value of about 1:10. We underline that many coincidence experiments with 1:10 S/N ratio value would be unsuited even only for checking the presence of the signal reliably. Instead, despite such a noticeable noise level, in the example discussed here the precise knowledge of the detector response allows one to construct the capture time spectrum according to Eq. (7) as shown in Fig. 6. The presence of the wanted physical signal manifests unambiguously and enables the determination of the reaction cross-section. 6. Summary and conclusion In this work, we have tested the feasibility of inclusive neutron production measurements by using a 4p thermalization counter in a low-intensity radioactive beaminduced reaction. In this kind of application, the detector is traversed by a central channel that contains the beam transport pipe and this, in turns, contains the reaction target. As verified by the simulations, the presence of this (almost empty) central channel makes the migration volume to be non-uniform with consequences on the neutron capture time distribution. Using a suitable 252Cf source we have investigated the capture time response of the detector by performing a 2-neutron inter-time measurement, a technique that results in an enhanced sensitivity to the occurrence of weak components with long capture times. The measured data are very nicely described by the superposition of 2 exponential components characterized by different mean capture times. Simulations performed with the GEANT 3.21 code corroborate the result of the measurement. The characterization of these 2 components in terms of their intensities and mean capture time according to Eq. (6) leads to the identification of a neutron energy of about 0.1 MeV above which the parameters tfast, tslow, qfast and qslow ¼ 1qfast are reasonably independent on the neutron energy (Fig. 5). With the example reported in Section 5 one can evaluate the importance of the capture time information in a neutron production measurement. There is evidence to conclude that the combination of a weak intensity ion beam with a 4p detector, like the Polycube, can make feasible experiments which would be otherwise impracticable.

6.5

Acknowledgements 6 0

0.0025

0.005

0.0075

0.01

ΔPnc/ Δtcapt (s−1) Fig. 6. Experimental capture time spectrum in the 8Li(4He,n)11B reaction at ECM ¼ 1.2570.2 MeV versus the capture time probability density (6). The straight line is the linear data fit according to Eq. (7). The background level is represented by the dashed line. The tcapt range involved is reported on the top.

The authors are truly indebted to Prof. R.W. Kavanagh for providing the Polycube neutron detector and for the many useful discussions and advice. References [1] A.C. Mueller, et al., Nucl. Instr. and Meth. B 56 (1991) 559. [2] D.J. Morrisey, et al., Nucl. Instr. and Meth. B 204 (2003) 90.

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