The Darmstadt High-Intensity Photon Setup (DHIPS) at the S-DALINAC

Nuclear Instruments and Methods in Physics Research A 640 (2011) 6–12

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

The Darmstadt High-Intensity Photon Setup (DHIPS) at the S-DALINAC d ¨ K. Sonnabend a,, D. Savran b,c, J. Beller a, M.A. Bussing , A. Constantinescu e, M. Elvers d, J. Endres d, a a d a a a ¨ ¨ , S. Muller , N. Pietralla a, C. Romig a, M. Fritzsche , J. Glorius , J. Hasper , J. Isaak , B. Loher d a a d ¨ , A. Zilges , M. Zweidinger a A. Sauerwein , L. Schnorrenberger , C. Walzlein a

Institut f¨ ur Kernphysik, TU Darmstadt, Schlossgartenstr. 9, D-64289 Darmstadt, Germany ExtreMe Matter Institute EMMI and Research Division, GSI Helmholtzzentrum f¨ ur Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt, Germany c Frankfurt Institute for Advanced Studies FIAS, Ruth-Moufang-Str. 1, D-60438 Frankfurt am Main, Germany d Institut f¨ ur Kernphysik, Universit¨ at zu K¨ oln, Z¨ ulpicher Str. 77, D-50937 K¨ oln, Germany e GSI Helmholtzzentrum f¨ ur Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 November 2010 Received in revised form 28 January 2011 Accepted 8 February 2011 Available online 23 March 2011

The Darmstadt High-Intensity Photon Setup (DHIPS) for photon-scattering and photodissociation experiments at the superconducting electron linear accelerator S–DALINAC has been improved. We report on its present performance. The developments include the installation of a segmented radiator for an online evaluation of the beam energy and the construction of a second photon-scattering setup with a Clover detector to deduce parity assignments. The enhancement of the existing photonscattering setup with an additional 100% HPGe detector and some minor changes are described as well as the installation of two setups for photon-induced activation. In addition, we present a comparison of experimental data to Monte Carlo simulations using the codes Geant 3.21 and GEANT4. & 2011 Elsevier B.V. All rights reserved.

Keywords: Photons Bremsstrahlung Nuclear astrophysics Nuclear structure

1. Introduction The study of photon-induced reactions is of importance for the investigation of low-spin nuclear excitation modes and, consequently, for the correct description of the nuclear physics input to several astrophysical scenarios. Real photons are a clean probe for nuclear physics experiments since, due to the well-known excitation mechanism, quantum numbers and electromagnetic matrix elements can be extracted from measured quantities in a model independent way [1]. Concerning nuclear astrophysics they can contribute to the determination of reaction rates relevant for the nucleosynthesis of heavy nuclei. This is most evident in the case of the nucleosynthesis of the so-called p nuclei. These proton-rich isotopes between Se and Hg cannot be synthesized by neutron capture processes and are thought to originate from explosive events lasting a few seconds and delivering temperatures of about 223  109 K [2,3]. One commonly accepted scenario at least for the production of the heavier p nuclei are the O-Ne-layers of type II supernovae in which s- and r-process seed nuclei are converted to the

 Corresponding author. Current address: Goethe-Universitat ¨ Frankfurt, Institut ¨ Angewandte Physik, Fachbereich 13 – Physik, Max-von-Laue-Str. 1, D-60438 fur Frankfurt am Main, Germany. Tel.: þ 49 69 798 47467; fax: þ49 69 798 47444. E-mail address: [email protected] (K. Sonnabend).

0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.02.107

proton-rich side of the valley of stability by a series of (g,n), (g,p), and (g, a) reactions [4,5]. In addition, the data of (g,n) reactions can be used to constrain the prediction of the neutron capture cross-sections of the branch point nuclei in the s process that determine the final isotopic abundances produced [6,7]. Some decades ago, (g,n) reactions were intensively studied in the energy range of the Giant Dipole Resonance (GDR) see, e.g., [8]. However, the photon sources available at that time did not allow to measure the reaction cross-section close to the neutron separation energy Sn which is the astrophysical relevant energy range. Nowadays, bremsstrahlung sources providing highly intense continuousenergy beams are available and make a strong impact on the development of nuclear astrophysics, e.g., at Darmstadt [9] and Dresden [10] or elsewhere [11]. These setups are used to perform experiments relevant for p-process nucleosynthesis [12–16] and s-process nucleosynthesis [6,17]. Comparable studies are also performed using quasi-monoenergetic photons produced by Laser Compton Backscattering (LCB) [18–20]. In the case of nuclear physics, experiments are performed at excitation energies below the particle thresholds using Nuclear Resonance Fluorescence (NRF) [21]. The combination of bremsstrahlung as photon source with high-resolution high-purity Germanium (HPGe) detectors for the detection of the emitted photons was widely used in the past decade [22–26]. Recently, LCB photons also provide the intensities to perform these studies [27–29].

K. Sonnabend et al. / Nuclear Instruments and Methods in Physics Research A 640 (2011) 6–12

However, bremsstrahlung is one of the major sources to study photon-induced reactions for both nuclear structure and nuclear astrophysics. At the Darmstadt High-Intensity Photon Setup (DHIPS) at the superconducting electron linear accelerator S–DALINAC [30], bremsstrahlung is produced by stopping the intense electron beam of the injector in a thick radiator. The narrow arrangement and, thus, the short distance between the target position and the radiator leads to very high intensities and allows the investigation of reactions even with small crosssections. To keep DHIPS up-to-date, extensions and improvements are continuously included. This paper summarizes the main developments of the last years. Section 2 describes the extensions of the existing setup while Section 3 compares the results of Monte Carlo simulation codes to experimental data. Section 4 explains different approaches to determine and monitor the electron energy E0 and, thus, the endpoint energy of the bremsstrahlung spectrum Emax at DHIPS. Finally, Section 5 provides ideas for further improvements.

2. Extensions of the existing setup The facility for real-photon scattering at the S–DALINAC was last described in Ref. [9] focussing on the effects of the replacement of the radiator target and collimator in order to allow higher beam energies of up to about 10 MeV. The use of copper for both parts ensures nearly neutron-free production of bremsstrahlung spectra up to the electron energies provided by the injector of the S–DALINAC (max. 10 MeV). In this section, we will present the extensions of several parts of the setup that have been included in the last years. An overview of the current setup is given in Fig. 1. For experiments on NRF a third detector (Det3) was integrated into the existing photon-scattering setup (T1, Section 2.2). A second scattering setup behind the first target position was installed to allow parity assignments (T2, Section 2.3) in parasitic mode. Additionally, two setups with different photon intensities are available to study photodisintegration reactions using the activation technique (T0 and T1, Section 2.4). The beam-spot size at target positions T1 and T2 was determined via a picture on radiographic film to be about 25 and

Det1

30 mm in diameter, respectively. Therefore, the NRF targets with a diameter of 20 mm are completely illuminated by the photon beam. All HPGe detectors are placed at a distance of about 25 cm to the targets—detectors 1 and 4 at an angle of 901 to the beam direction, detectors 2 and 3 at 1301, respectively. The NRF targets usually consist of powder pressed into a cylindrical plastic container and are sandwiched between boron targets that are used for normalization (see, e.g., Ref. [1]). Their orientation to the beam direction is chosen such that the different detectors examine the target under comparable conditions. 2.1. Segmented radiator The copper radiator described in Ref. [9] has been replaced by a segmented radiator consisting of four copper plates each of 3 mm thickness and about 10 cm in diameter to enable energy deposition of up to 300 W as caused by a 30 mA electron beam of 10 MeV energy. While the electron beam is still completely stopped within the radiator the amount of charge deposited in each single copper plate depends on the energy of the electron beam. The plates are electrically isolated by and mounted on ceramic discs in order to read out the deposited charges separately which allows an online monitoring of the electron energy (see Section 4.2). 2.2. Extension of photon-scattering setup The setup at the first target position (T1, Fig. 1) is optimized to perform high-resolution NRF experiments using HPGe detectors for g-ray spectroscopy. In order to improve the overall performance, the setup was extended by a third large volume HPGe detector of 100% relative efficiency (Det3) similar to the two already available detectors (Det1, Det2). In addition, each detector is now equipped with a bismuth germanate (BGO) shield for active escape suppression. The active suppression considerably improves the peak-to-background ratio at medial energies and strongly diminishes single and double escape peaks [31]. Both, suppressed and unsuppressed spectra, are taken in parallel using individual peak sensing 8k ADCs in order to minimize dead times. The upper panel of Fig. 2 shows two spectra of a NRF experiment on 136Xe at an endpoint energy Emax ¼ 9100 keV [24]. The Compton and escape suppression by means of a BGO detector reduces the background in the spectrum considerably, e.g., the single escape (SE) peak around 5 MeV is suppressed by a

counts /keV

Det2

radiator e-

T1

Det3 1m

103

102 without BGO with BGO

T2

T0

Det4

Fig. 1. The extended Darmstadt High-Intensity Photon Setup (DHIPS) at the S–DALINAC. The electron beam of up to 10 MeV is stopped in a segmented radiator to produce bremsstrahlung. Two experimental sites, T1 and T2, are available for NRF experiments. Photon-induced activation is performed at target positions T0 and T1, respectively. For details, see text.

7

2.0 1.8 1.6 1.4 1.2 1.0

ratio sensitivity limit with/without BGO

4500

5000

5500

6000 6500 E (keV)

7000

7500

Fig. 2. Upper panel: spectra measured with and without Compton and escape suppression by means of a BGO detector for the reaction 136 Xeðg, g0 Þ. Lower panel: corresponding gain in sensitivity based on the spectra with and without BGO suppression, respectively (compare Ref. [32]).

8

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factor of 4. The sensitivity limit of a given experiment as defined in Ref. [32] depends on the background present in the corresponding spectrum, therefore, the suppression leads to a higher sensitivity. The gain in sensitivity for the 136Xe experiment using the BGO escape suppression is shown in the lower part of Fig. 2. The improvement of the sensitivity limit by a mean factor of about 1.4 clearly proves the importance of an effective BGO escape suppression. The BGO shields are placed in large lead collimators that are included in the massive lead shielding surrounding the detectors. Because of the close geometry of DHIPS this massive shielding is mandatory to suppress background stemming directly from the radiator and, thus, to ensure low-background spectra. 2.3. Parity assignment setup Parity assignments using the NRF method in combination with Compton polarimetry of the scattered g-rays require rather high statistics or a significant increase in polarization sensitivity especially at energies above Eg ¼ 4 MeV [1]. Previously, such experiments have been performed at DHIPS at the first target position (T1, Fig. 1) using a segmented HPGe detector [33]. However, since the intensity of the photon beam is nearly unaffected by a typical NRF target we constructed a second target position (T2, Fig. 1) to reuse the transmitted photon beam to perform these time-consuming parity measurements parasitic to NRF or activation experiments performed at the first target position. This second setup has been equipped with a HPGe Clover detector but also other segmented HPGe detectors might be installed at this position (Det4). Fig. 3 shows the deduced polarization sensitivity Q of the used Clover detector that was determined in Ref. [34] up to a photon energy of 9 MeV. These results prove the feasibility to run experiments on parity assignments at target position T2. Since the experiments do not disturb measurements at the first target position T1, this additional setup greatly enhances the possibility to determine parities at DHIPS. 2.4. Description of activation setups The high-intense bremsstrahlung available at DHIPS allows to perform activation experiments down to energies close to the reaction threshold. However, to extract information on the absolute value of the reaction cross-sections it is mandatory to know the energy dependence of the bremsstrahlung intensity as well as its absolute value (compare Eq. (1)). While the energy dependence can be simulated with high accuracy using Monte Carlo codes (compare Section 3) the absolute value has to be determined during the activation. At target position T1 in Fig. 1, data from photon-scattering off 11 B can be used to normalize the photon intensity. As described in, e.g., Refs. [35,36], the g transitions in 11B enable to monitor the photon intensity in the energy range of 2.2–8.9 MeV by analyzing

12 Q (%)

8 4 0 2000

3000

4000

5000 6000 E (keV)

7000

8000

9000

Fig. 3. Measured polarization sensitivity Q of a HPGe Clover detector up to 9 MeV photon energy. The dashed line is a fit as explained in Ref. [34].

the corresponding peaks in the NRF spectra measured parallel to the activation. Thus, sandwich targets consisting of the 11B normalization standard and the targets to be activated have to be used at this target position. Due to the available photon intensities of about 106 keV  1 cm  2 s  1 at 0:7  Emax , a typical activation measurement—like 197 Auðg,nÞ—with an amount of 150 mg target material takes about 24 h of beam-time close to the reaction threshold Sn to produce a sufficient reaction yield for the following offline g spectroscopy. If the natural abundance of the isotope to be investigated is very low, like in the case of 196Hg with only 0.15%, the demand of highly enriched target material can be attenuated using the 1 higher photon intensities of about 3  108 keV cm2 s1 available at target position T0 (see Fig. 1). However, the reaction 11 Bðg, g0 Þ cannot be used for normalization since there is no space available to place and shield an additional HPGe detector properly against the high beam-induced background. Though, a normalization can be done by activating additional targets in parallel that include isotopes with precisely known (g,n) cross-sections. Such activation standards are the reactions 197 Auðg,nÞ [14,37] and 187 Reðg,nÞ [38,39] that were established at target position T1. The higher photon intensities at target position T0 allow not only to measure reactions based on very low amounts of target material down to several mg. In addition, the simultaneous activation of several targets is possible. Therefore, the photon beam can be used very efficiently to perform systematic studies of ðg,nÞ reaction cross-sections in a broad mass range.

3. Simulations vs. experimental data The characteristics of the bremsstrahlung setup and the detectors were extensively simulated using the Monte Carlo code GEANT4 [40,41], version 4.9.0.p01 with the Standard packages. The reliability of these simulations for the production and behavior of photons in the energy range of 3–10 MeV was not stated yet so that the results of the simulations were compared to detailed experimental data. In general, we found improved agreement compared to the Monte Carlo code Geant 3.21 [42]. 3.1. Energy dependence of bremsstrahlung spectrum The knowledge of the energy dependence of the produced bremsstrahlung spectrum is of crucial interest especially for the activation experiments carried out at DHIPS. Here, the distribution near the endpoint energy is most important due to its high impact on the yield of unstable isotopes. However, former simulations for target position T1 (see Fig. 1) using Geant 3.21 had to be corrected for Eg Z0:8  Emax since, e.g., photon energies higher than the electron energy E0 were produced. The correction is based on experimental data as described in detail in Ref. [35]. Performing simulations with the same geometrical settings using GEANT4 yields results equivalent to the corrected Geant 3.21 simulations. Thus, the results of GEANT4 agree well with experimental data without any further correction. This is especially useful since it was found in Ref. [43] that the correction derived for setup T1 is not valid for setup T0 in front of the copper collimator (see Fig. 1). Experimental data on the standard reactions 197 Auðg,nÞ and 187 Reðg,nÞ pointed to a less severe correction than at setup T1. Fig. 4 shows the discrepancies between the different simulations at setup T0. The simulation derived with GEANT4 yields a distribution between the ones allocated by Geant 3.21 (original and corrected) as expected in Ref. [43]. An experimental test of the distribution at T0 is provided by the comparison of the integrated cross-sections Is of isotopes with different neutron separation energies Sn like, e.g., 197Au and

K. Sonnabend et al. / Nuclear Instruments and Methods in Physics Research A 640 (2011) 6–12

103 102 10

Geant 3.21-original Geant 3.21-corrected GEANT4

7500

8000

8500 9000 E (keV)

9500

10000

Fig. 4. Simulated bremsstrahlung distributions at target position T0 (see Fig. 1) for an electron energy of E0 ¼ 9900 keV. The results from original and corrected Geant 3.21 simulations and GEANT4 simulations are shown. See text for details.

1.5

f

1.0 0.5

Geant 3.21 - original Geant 3.21 - corrected GEANT4

0.0 8000

8500

9000 Emax (keV)

9500

10000

Fig. 5. Normalization factor f as a function of endpoint energy Emax. See text for details.

187 Re with Sn ¼ 8071 and 7359 keV, respectively. In both cases, the (g,n) cross-sections are known with high accuracy down to the reaction threshold. The available data and a parametrization of the cross-section of 197Au from the reaction threshold up to the GDR are summarized in Ref. [37]. Concerning 187Re, the measurements using continuous-energy bremsstrahlung suit well to older data (see Ref. [39]) and are in good agreement to recent data derived from experiments using LCB photons [38]. The yield Y of an activation measurement can be determined by offline high-resolution g spectroscopy and is given by

YðEmax Þ ¼ NT  Is ðEmax Þ Z Emax sðEg Þ  Ng ðEg ,Emax Þ  dEg ¼ NT 

ð1Þ

Sn

with the number of target nuclei NT, the integrated cross-section Is , the (g,n) cross-section sðEg Þ, the bremsstrahlung distribution Ng ðEg ,Emax Þ, and the maximum photon energy Emax [13]. If the energy dependence and absolute value of the cross-sections is known, a normalization factor f defined as YðReÞ NT ðAuÞ  Is ðAuÞ  f¼ YðAuÞ NT ðReÞ  Is ðReÞ

ð2Þ

should be independent of Emax and close to unity if the energy dependence and absolute value of the bremsstrahlung distribution is simulated correctly. The values of f for five energies Emax using the different types of simulations are shown in Fig. 5. For the distribution derived with Geant 3.21, the normalization factor f decreases to unity with increasing values of Emax, thus, pointing to an overestimation of the real bremsstrahlung distribution close to the endpoint energy. In contrast, f increases with increasing Emax if the corrected Geant 3.21 simulation is used. Because f differs significantly from unity for all energies Emax, a large underestimation of Ng ðE,Emax Þ can be derived in that case. Finally, the evaluation using the results of the GEANT4 code yields values compatible with unity within an uncertainty of 12% independent of the observed Emax. Therefore, the bremsstrahlung distribution at target position T0 is simulated correctly by the implemented geometry in GEANT4.

In the previous section we have shown that the shape of the produced bremsstrahlung spectrum for a given endpoint energy is well reproduced by GEANT4 Monte Carlo simulations and, therefore, the absolute normalization of the integrated photon flux can be done for the whole energy range using different reference reactions. However, the photon flux near the endpoint energy Emax strongly depends on the absolute value of the endpoint energy. This energy region is especially important for measurements of (g,n) cross-sections near the neutron separation energy Sn. As described in Section 3, the yield of an activation experiment is mainly defined by the integral over the product of the photoneutron cross-section sðEg Þ and the time-integrated photon flux Ng ðEg ,Emax Þ (see Eq. (1)). As an example Fig. 6 shows the expected yield for the (g,n) reaction on 197Au as a function of the endpoint energy of the bremsstrahlung spectrum Emax. The yield strongly depends on the endpoint energy especially near the neutron separation energy Sn. An accurate extraction of information on the photo-neutron cross-section sðEg Þ from the reaction yield relies on a proper knowledge of the endpoint energy Emax. For this kind of experiments it is therefore mandatory to determine and monitor during the whole experiment the absolute endpoint energy Emax which is given by the incident electron energy E0. In the first part of this section, we describe the determination of the absolute value of the electron energy using different methods which are however not applicable to monitor the electron energy during the experiment. In the second part of this section, a new online monitor for the value of the incident electron energy using the segmented radiator is presented. 4.1. Absolute determination of the electron energy The conventional method to determine and adjust the electron energy at DHIPS is to analyze the momentum of the electron by the magnetic field of a calibrated deflection magnet placed in front of the radiator target. Within the dipole magnet the electron beam is bent by an angle of 403 into a second beamline. The bending angle can be determined with high precision using a beam position target about 4 m downstream this second beamline. By measuring the magnetic field B needed to bend the beam by 403 , the energy E0 of the electrons can be determined by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! e2 r2 2 2 E0 ¼ m0 c B þ11 ð3Þ m20 c2 where m0 denotes the electron rest mass and r the effective bending radius. In order to determine the effective bending radius including fringe fields and to study the influence of a finite beam-spot size, the particle tracks inside the magnet were simulated. In a first step, the magnetic field was calculated in three dimensions with the software package CST EM STUDIOTM [44] which uses the

1 0.1 10-2 10-3 10-4

Sn = 8071 keV

4

Y (a.u.)

10

4. Determination of electron energy

E0 = 9.9 MeV

N (E) (a.u.)

105

9

8100

197Au

8300

8500 Emax(keV)

8700

(,n)

8900

Fig. 6. Expected yield of the reaction 197 Auðg,nÞ as a function of the endpoint energy Emax of the simulated bremsstrahlung spectrum. To calculate the yield the parameterization of the reaction cross-section given in Ref. [37] has been used.

K. Sonnabend et al. / Nuclear Instruments and Methods in Physics Research A 640 (2011) 6–12

design of the magnet as input. In a second step, the extracted magnetic field was imported in a GEANT4 simulation to track the electron orbits inside the field. Using the particle tracks, the effective bending radius of the dipole magnet can be calculated. The effects of a finite beam-spot size turn out to be about or less than 10 keV and together with the precision of the beam position target 4 m downstream the beamline, an accuracy of about 50–100 keV in the energy determination can be achieved. However, for a precise absolute determination of the electron energy a proper alignment of the incoming beam and an accurate measurement of the field strength are mandatory. To verify the results of the simulations, we determined the absolute electron energy for a few settings in an alternative way. This was done by measuring the spectral distribution of the produced photon beam using a large volume HPGe detector in order to determine the endpoint energy of the bremsstrahlung spectrum Emax. The detector was placed behind the collimator at target position T1 (see Fig. 1). The electron beam current was reduced far below the sensitivity of the available analyzer to limit the count rate in the HPGe detector to a few kHz in order to avoid pile-up events. Spectra were taken for an electron energy of Econv ¼ 7600 keV determined 0 using the conventional method described above. Fig. 7 shows the measured photon spectrum for Econv ¼ 0 7600 keV. Pile-up events were suppressed by using the veto of the spectroscopy amplifier, however, pile-up events could not be completely avoided. Also a minor contribution of high energetic cosmic radiations are present in the spectrum above the energy of the electron beam E0. To account for these small fractions near the endpoint energy of the bremsstrahlung distribution Emax, the values at high energies (Eg Z8000 keV) were linearly extrapolated to lower energies and subtracted from the spectrum (see Fig. 7). The cutoff at an energy around 7.6 MeV is clearly visible. Since the shape of the bremsstrahlung distribution close to Emax is very sensitive to the exact value of E0, we compared the measured spectrum to simulated spectra of various energies in order to determine the electron energy E0. The spectra corresponding to different energies Esim were simulated with GEANT4 0 taking into account that the measured spectrum is a convolution of the incoming photon spectrum and the photoresponse of the HPGe detector. Simulated and measured spectra were normalized to the number of events in the energy region 7000–8000 keV. The comparison was performed by binning the spectra in equal 10 keV wide energy bins and calculating

a¼

n Nisim Niexp 1X n i ¼ 1 DNisim þ DNiexp

DNisim and DNiexp represent their uncertainties, respectively. Fig. 8 shows the resulting a for Econv ¼ 7600 keV. 0 As can be seen, the values of a reach a clear minimum at around 7.65 MeV. For a more precise determination of the minimum, we fitted a quadratic polynomial to the data points. The resulting position of the minimum of about 7.64 keV represents the determined endpoint energy of the measured spectrum. This value is about 40 keV higher than that expected from the measurement of Econv and, thus, is within the stated accuracy of 0 the measurement. Using this method, different settings of the deflecting magnet used for the previously discussed energy determination can be calibrated. Thus, the accuracy of this former method is significantly improved and based on an experimental determination of the endpoint energy of the produced bremsstrahlung spectra.

4.2. Online energy monitor The above described methods of determining the electron energy require either a deflection of the electron beam or its reduction to a very low intensity. Thus, both methods require an interruption of the running experiment and, therefore, are not suitable for a permanent control of the beam energy E0. For the purpose of an (non-interfering) online monitoring, we redesigned the radiator. As described in Section 2.1, the new radiator consists of four electrically isolated 3 mm thick copper plates. Since the penetration depth of the impinging electrons depends on their energy, the charge deposited in each copper plate varies for different energies. At electron energies in the region of 6–10 MeV most of the beam is stopped within the first two plates. Therefore, we choose the ratio Q2/Q1 of the charges deposited in the first (Q1) and second (Q2) plate to measure the mean penetration depth and, thus, the beam energy. The left part of Fig. 9 shows the ratio Q2/Q1 as a function of time for two different but very similar electron energies of Econv ¼ 8250 and 8300 keV. For each value the 0

α

10

!2 ð4Þ

calculated α 7550

in the energy region 7000–8000 keV for each simulated spectrum. The values Nsim and Nexp represent the number of counts in the i i energy bin i of the simulated and experimental spectra while

Q2/Q1

102

7600

1.4

Econv = 7600 keV 0

103

polynomial fit 7650 E0 (keV)

7700

7750

Fig. 8. Calculated a between the simulated and the measured spectrum with Econv ¼ 7600 keV. The dotted line represents the fit of a quadratic polynomial with 0 its minimum at about 7640 keV.

104 counts / 10 keV

3.0 2.5 2.0 1.5 1.0 0.5 0.0

1.4

Econv = 8250 keV 0

1.3

1.3

1.2

1.2

10 Econv = 8300 keV 0

1.1 1 6800

7200

7600 E (keV)

8000

8400

Fig. 7. Photon spectrum measured with a HPGe detector placed directly in the bremsstrahlung photon beam with Econv ¼ 7600 keV. The small amount of cosmic 0 and pile-up events (dotted line) has been determined by a linear fit in the energy region 8000–9000 keV and subtracted from the spectrum for the further analysis.

1000

3000

5000 t (s)

7000

1.1 9000

Fig. 9. Measured ratio Q2/Q1 as a function of time for two different energies. Each value is averaged over a time period of 10 s. The resulting histograms of the measured values Q2/Q1 (right panel) prove that even for the small relative energy difference of only 50 keV a clear separation of the distributions is possible.

K. Sonnabend et al. / Nuclear Instruments and Methods in Physics Research A 640 (2011) 6–12

7

Q2/Q1

5 3 1 6000

6500

7000

7500 E0 (keV)

8000

8500

9000

Fig. 10. Calibration of the ratio Q2/Q1 as a function of electron energies.

charges have been integrated for 10 s in order to minimize the influence of high frequency noise. Even though the two energies differ only by 50 keV a clear difference in Q2/Q1 is observed. The right part of Fig. 9 shows histograms of the values plotted in the left part. Clearly, relative changes in energy of less than 25 keV are easily noticeable proving the high sensitivity of this method. Unfortunately, an absolute determination exclusively based on this method is not reliable. Therefore, we use it in combination with the two previously described methods by calibrating the ratio Q2/Q1 as a function of the electron energy determined using the methods described in Section 4.1. Fig. 10 shows such a calibration. The errorbars determined by the FWHM distributions shown in the right part of Fig. 9 are smaller than the symbols. While this necessary calibration is a small drawback of the method it is by far outweighed by its advantages: The method allows a monitoring of the electron energy during the entire experiment without causing any disturbance and it is in addition applicable for intensities of only a few nA up to the maximum currents of many mA available at DHIPS.

5. Conclusion The Darmstadt High-Intensity Photon Setup (DHIPS) has been extended during the past years with additional measuring setups and new features. A second setup (T2, Fig. 1) to perform timeconsuming experiments on parity determination using Compton polarimetry is equipped with a Clover (or similar) detector and located downstream the well-established first setup for NRF experiments (T1, Fig. 1). This setup T1 was extended by another HPGe detector to increase the efficiency for NRF experiments while all three HPGe detectors at this setup are now equipped with BGO shields for active background suppression to improve the peak-to-background ratio in the measured spectra. In addition, setup T1 can also be used for photo-activation experiments with the reaction 11 Bðg, g0 Þ serving as normalization standard for the absolute photon intensity. Upstream this setup, in front of the copper collimator and directly behind the radiator a second setup for activation experiments (T0, Fig. 1) is located benefitting from photon intensities enhanced by a factor of about 300. In most cases, all three setups can be used simultaneously and, therefore, increase the number of results obtained in one beam-time. Especially the activation experiments rely on a detailed knowledge of the electron energy E0 since this determines the endpoint of the produced continuous-energy bremsstrahlung spectrum Emax. Therefore, a segmented radiator target was developed and calibrated to monitor the beam energy online with an accuracy of about 30 keV. The energy dependence of the bremsstrahlung spectrum was extensively studied comparing simulations based on the Monte Carlo code GEANT4 to experimental data. In future, an upgrade of the injector of the S–DALINAC is planned to enable higher electron energies up to 14 MeV as well

11

as higher currents up to 250 mA. The most challenging part to utilize these features at DHIPS is the reduction and/or handling of neutrons produced by high-energy bremsstrahlung. Up to 13 MeV the usage of aluminum instead of copper avoids this problem. However, the lower density and proton number results in a larger radiation length for electrons and a smaller absorption coefficient for photons so that the dimensions of the radiator target as well as the collimator would have to be increased. Once this problem is solved, the higher energies will allow to measure ðg,nÞ reaction cross-sections in the mass range A  100. The combination with higher beam currents, and therefore photon intensities, enables to measure also (g,p) and ðg, aÞ reaction crosssections in selected cases using the activation technique. Concerning NRF experiments, higher energies allow to broaden the observed energy range and measure also the photo-response of nuclei in the region of the N¼Z¼ 28 shell closure, such as 54Fe and 60 Ni, up to the neutron separation threshold Sn. To complement the measurements with high-intense bremsstrahlung a high-resolution low-energy photon tagging setup, the NEPTUN facility [45,46], was developed and constructed at the S–DALINAC. It provides a unique opportunity to study the complete photoresponse with a focus on astrophysically relevant energies based on investigations with real photons. However, the studies mentioned so far are limited to stable isotopes while in nuclear structure as well as nuclear astrophysics the investigation of unstable and exotic nuclei becomes more and more requested which is possible, e.g., at the SIS/FRS/LAND setup at GSI, Darmstadt, using Coulomb excitation and dissociation in inverse kinematics [47–49].

Acknowledgments We thank R. Eichhorn and the accelerator group of the S–DALINAC for their support during beam-times. The support of S. Volz during the improvement of the setup is also acknowledged. This work was supported by the Deutsche Forschungsgemeinschaft (SFB 634, ZI510/4-1), by the LOEWE program of the State of Hesse (Helmholtz International Center for FAIR), and by the Alliance Program of the Helmholtz Association (HA216/EMMI). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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