Background gamma measurement for low energy astro-physical reaction at FRENA

. The majority of astrophysical reactions have very low cross-sections (nanobarn to picobarn range) and significant error bars [1, 2]. Hence γ -ray background studies up to 3 MeV have been performed inside the Facility for Research in Experimental Nuclear Astrophysics (FRENA), Saha Institute of Nuclear Physics, Kolkata, India. A meticulous background calculation has been performed at di ff erent positions inside the accelerator hall and beam hall using two NaI(Tl) detectors [3]. An XPS measurement has been carried out to identify elements present in the FRENA wall concrete. Activity calculations done in [4] shows that neutrons produced by various experiments may combine with any nearby elements and form various radioactive isotopes with long half-lives, which may produce a γ -ray background .


Introduction
Gamma-ray measurement especially those dedicated to the study of astrophysical reactions, require a close attention to measuring and minimizing the radiation background. Most astrophysical reactions occur far below the coulomb barrier, which results in very low crosssections (order of nanobarn to picobarn) with significant error bars [1,2]. Therefore, the precious cross-section measurements from γ-spectroscopy greatly depend on the γ-ray background. A detailed background measurement will also help to minimise the uncertainty in the observed data.
Background γ-rays having energy less than 3 MeV, comes mainly from naturally present radioactive isotopes, energy higher than 3MeV are mainly due to the cosmic ray interaction at the upper atmosphere. In astrophysical studies low energy γ-rays play major role. An active shielding of higher energy γ-ray spectrum has been done by sophisticated techniques in data acquisition and detector systems. For the study of higher energy γ-ray spectrum and the rejection of the backgrounds, underground labs are preferable [5].
Facility for Research in Experimental Nuclear Astrophysics (FRENA) is an upcoming tandem accelerator located at Saha Institute of Nuclear Physics (SINP), Kolkata, India. FRENA is a high current and low energy accelerator, which is primarily designed for nuclear astrophysics studies. The accelerator can operate with the terminal voltage of 0.2 to 3 MV with the standard voltage resolution of 3 × 10 −5 (∆E/E) and the typical DC current at 3 MV terminal voltage for the H + ion 300 µA, for the He 2+ ion is 100 µA and for the heavier ions between 20-60 µA [6].
In this work, background measurements of the γ-ray spectrum below 3 MeV, most relevant to nuclear astrophysics study, have been performed. It primarily originate from radioactive naturally occurring isotopes. A details counts per seconds(CPS) measurement have been performed. Two NaI detectors have been used for the background measurements due to their higher efficiency [3]. An XPS analysis of the indoor wall concrete has been conducted for elemental search.

Sources of γ-ray background
The earth's crust is composed of a variety of elements and isotopes with different abundances [7]. These naturally occurring isotopes continuously produce lower energy γ-ray. This represents the background for any laboratory performing lower energy γ-ray spectroscopy. These γ-ray spectrum need to be measured precisely and to reduce background effects, detectors need to be passive shielding with high Z materials (Pb, Z=82).
The elements potassium, lead, thorium, and uranium are abundantly found in nature. Natural potassium has 0.012% of 40 K, which emits γ-rays with an energy of 1.460 MeV and a half-life of 1.248×10 9 years. 204 Pb decays through a long chain of gammas and alpha particles, effectively producing γ-ray with energies of 899.15 keV and 911.74 keV with relative intensities of 99.17% and 91.5%. 227,232 Th and 235,238 U have a long gamma chain with a wide energy range, also Gd, Dy, Ta, and Tl have a large number of isotopes with larger abundances and emit γ-rays.  [7] .
The existence of C, O, Si, Ca, Mg, Al, Ta, P, and Ce in the concrete powder of indoor blocks has been verified by comparing experimental X-ray photoelectron spectroscopy(XPS) data with NIST database (Figure 1). The primary components of concrete are C, O, Si, Ca, H, and Mg and K, Ca, Si, Mg and O are the rock forming elements (Figure 2).
At FRENA, the gamma background has been measured under beam-off conditions. While the accelerator is operating, the neutron emitted from an experiment may interact with nearby stable elements (elements of concrete and machine parts) and may produce a long-lived activity. Which may be a source of gamma background for the experiments being performed close-by [4].

Experimental details 3.1 Venues of the measurement
The FRENA building has 1.2 m thick walls, which effectively isolate the indoor and outdoor environments from radiations. However, naturally occurring radioactive isotopes ( 40 K, 211 Pb, 214 Bi and so on) in the wall concrete itself contribute to the gamma-ray background in the experimental measurements performed in the FRENA building. The same concrete wall divides the beam hall from the accelerator hall. The roof is made of 60 cm thick concrete to reduce cosmic background radiation [5]. The background measurements has been performed at positions A and B, 132 cm and 265 cm from the concrete wall, respectively, in the accelerator hall and the beam hall ( Figure 3).

Detector, Power supplies and data acquisition system
Two 1.5"×1.5" (diameter×length) NaI(Tl) crystal scintillator detectors have been used. Both have glass photomultiplier tubes. Detectors numbered detector 1 and detector 2. A specific power supply has been used for each detector.
For detector 1, the SENS-TECH PM30D power supply has been used. The detector bias was given 2100 V and the leakage current was 60 µA. For detector 2, Canberra 3002D power supply has been used. The bias supplied to the detector was 2000 V and the leakage current was 1.36 mA. Both the detectors have been placed at both positions alternatively. 60 Co was used as a known gamma source for the calibration of the detectors.
CAEN-DT5730 is an 8-channel 14-bit @ 500 MS/s digitizer used for the data acquisition system. CoMPASS multiparametric DAQ software was used to run the digitizer with DPP-PHA firmware.

Experimental setup
At positions A and B, detectors 1 and 2 have been placed (Figure 3). After a long run with bare detectors calibration has been performed by 60 Co (γ-ray energies, 1173 keV and 1332 keV). The different channels of the digitizer have concurrently been connected to detectors 1 and detector 2. The data were stored in a computer by CoMPASS multiparametric DAQ software. The background was measured in both spots by switching between the two detectors with their fixed power sources.
The detectors were shielded with lead bricks (3"×2"×1") after a long run with bare detectors to compare counts per second. In addition to these two locations, background measurements have also been made in the control room, detector lab, and close to the entrance gate of the FRENA building.

Results and Discussions
The background γ-ray spectrum at various energy ranges has been identified after a run of more than 7 days for background measurement. The notable peaks at the spectrum were mainly coming from isotopes of thorium, bismuth, lead, and potassium. From the calculation, 40 K has a background effect of roughly 72 counts per hour at 1.460 MeV of energy [8].
The possible isotopes from which the contribution to the background is significant are listed in Table 1. The energy range defined in Table 1 is determined by considering peak position and its energy uncertainty from Full-width Half Maximum (FWHM) fitting. The value is calculated for each peak energy range as peak value±(FWHM/2). The range is given only for data collected from measurements done in the beam hall, and during the study the Compton backscatter peak was ignored. Possible isotopes and their strongest γ-rays with their relative intensities have been compared to the JAERI-Data/Code 98-008 [8]. A count drop by a factor of 100 was observed for detectors shielded with Pb bricks(3"×2"×1") with respect to bare detectors. It confirms that gamma-ray background is coming from external elements, not due to any internal activity of the detectors.   The background measurement was done while the machine was off. Now, neutrons produced during experiments can interact with 181 Ta and form 182 Ta, which has a half-life of Table 1. The contribution of background γ-ray energy spectrum from the possible isotopes of sufficient relative γ intensity(%).