Facility for gamma irradiations of cultured cells at low dose rates: design, physical characteristics and functioning

Applied Radiation and Isotopes 115 (2016) 227–234

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Facility for gamma irradiations of cultured cells at low dose rates: design, physical characteristics and functioning Giuseppe Esposito a,b, Pasquale Anello a, Ilaria Pecchia a, Maria Antonella Tabocchini a,b, Alessandro Campa a,b,n a b

Health and Technology Department, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Roma, Italy INFN Roma1, Gruppo Collegato Sanità, Viale Regina Elena 299, 00161 Roma, Italy

H I G H L I G H T S








A gamma irradiation facility for chronic exposures of cells was set up at the Istituto Superiore di Sanità. The dose rate uniformity and the percentage of primary 662 keV photons on the sample are greater than 92% and 80%, respectively. The GEANT4 code was used to design the facility. Good agreement between simulation and experimental dose rate measurements has been obtained. The facility will allow to safely investigate different issues about low dose rate effects on cultured cells.

art ic l e i nf o

a b s t r a c t

Article history: Received 4 August 2015 Received in revised form 9 June 2016 Accepted 18 June 2016 Available online 24 June 2016

We describe a low dose/dose rate gamma irradiation facility (called LIBIS) for in vitro biological systems, for the exposure, inside a CO2 cell culture incubator, of cells at a dose rate ranging from few μGy/h to some tens of mGy/h. Three different 137Cs sources are used, depending on the desired dose rate. The sample is irradiated with a gamma ray beam with a dose rate uniformity of at least 92% and a percentage of primary 662 keV photons greater than 80%. LIBIS complies with high safety standards. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Gamma irradiation facility Chronic exposures Cell cultures GEANT4 simulations EBT3 GafChromic films

1. Introduction Ionizing radiation has always been naturally present in our environment. It comes from outer space, from the ground, and even from within our own bodies. It is present in the air we breathe, in the food we eat, in the water we drink, and in the construction materials we use to build our homes. Exposure of the population to ionizing radiation at low dose rates is therefore unavoidable (Hutchison and Hutchison, 2004; IAEA, 2005; Khan et al., 2012). Furthermore, by now the medical use of radiation has become an important part of modern healthcare, both for diagnostic and for therapeutic procedures. However, notwithstanding n Corresponding author at: Health and Technology Department, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Roma, Italy. E-mail addresses: [email protected] (G. Esposito), [email protected] (P. Anello), [email protected] (I. Pecchia), [email protected] (M.A. Tabocchini), [email protected] (A. Campa).

http://dx.doi.org/10.1016/j.apradiso.2016.06.018 0969-8043/& 2016 Elsevier Ltd. All rights reserved.

the clear clinical advantages, there is some risk of adverse radiation effects. The computation of the risk of detrimental health effects of ionizing radiation at high doses is now well established; in contrast the evaluation of the effects of low doses, or of prolonged exposure at low dose rates, has not yet reached the same level of knowledge (see, e.g., (Morgan and Bair, 2013) and references therein). In this context, the study of the biological effects induced by the exposure to ionizing radiations at low doses and at low dose rates is of great importance for public health. Specifically, it may have implications for the applicability of the linear no threshold model in extrapolating radiation risk data to the lowdose region (ICRP, 2007). Carrying out long time experiments at very low doses/dose rates is, at present, a hot topic for the scientific radiation protection research community. Radiobiological responses in these exposure scenarios have been scarcely investigated so far, primarily due to the lack of facilities allowing this type of exposure in controlled conditions. For in vitro studies on the radiation induced

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damage in cellular systems following protracted exposure to ionizing radiation, experiments must be done under well controlled conditions: dose and dose rate are to be measured with great accuracy and kept as uniform as possible on the sample; physiological conditions must be guaranteed for the duration of the experiment, that can last days or even several weeks, by maintaining the proper values for parameters such as temperature, humidity and CO2 concentration. Nowadays, some facilities for exposure of samples at different dose rates are already in operation. Among them, the Canadian Nuclear Laboratories apparatus, the Sandia gamma facility and the Tunisian facility are used for calibrating nuclear radiation instrumentation, irradiating personnel dosimetry badges, space technology development, military systems vulnerability testing, nuclear reactor component development, preservation of foodstuff, sterilization of medical devices. They are not used for radiobiological experiments. In Europe there are two irradiation facilities for irradiation of cells at low dose rates, one at the Stockholm University (SU), Sweden, and another at the Public Health England (PHE), UK. In this paper we describe the design, physical characteristics and functioning of a new facility; it nicely complements the two already existing, since it presents several innovative important features. They can be summarized in the following points: i) the Cesium sources are inside the cell culture incubator; ii) the whole range of possible dose rates is obtained without the use of filters; iii) it allows to perform irradiation experiments with cell cultures at dose rates down to 2 μGy/h , i.e., only one order of magnitude higher than the natural background. The first two points are important because they allow to change as little as possible the energy of the photons incident on the cell sample. This could be relevant because the variation of the energy also involves a variation of the photon beam quality and, possibly, of its biological effectiveness (see, e.g., (Hunter and Muirhead, 2009)). The third point is even more important since it allows to obtain radiobiological data on cell cultures over a range of dose rates up to now inaccessible for the experiments. Our facility, called LIBIS (Low dose/dose rate gamma Irradiation facility for in vitro BIological Systems) was set up at the Istituto Superiore di Sanità (ISS), Italy. It allows the exposure, inside a CO2 cell culture incubator, of cultured cells to low gamma doses at a dose rate ranging from few μGy/h to some tens of mGy/h. Among the limited data sets dealing with low/very low dose rate effects, we want to mention the experiments carried out at the Stockholm University facility using endothelial cells (HUVEC) that have been chronically exposed to dose rates of 1.4, 2.4 and 4.1 mGy/h for 1, 3, 6 and 10 weeks. The results have shown modulation of proinflammatory response, premature senescence and associated proteomic changes (Yentrapalli et al., 2013a, 2013b; Ebrahimian et al., 2015). Other studies were carried out for period of times lasting some months in an extremely low radiation environment at the underground Gran Sasso National Laboratory of the National Institute of Nuclear Physics (INFN), Italy. These long term experiments on cultured cells have shown that environmental radiation contributes to the development and maintenance of cellular defense mechanisms (Fratini et al., 2015). Use of LIBIS can help to investigate the dose rate dependence of the capability of in vitro biological systems to develop stress response mechanisms. Finally, LIBIS may be used in the field of biodosimetry, allowing to extend investigation of the dose rate dependence of micronuclei induction and chromosome aberrations (Bhavani et al., 2014; Bakkiam et al., 2015; Tamizh Selvan et al., 2015) to very low dose rate values.

2. Design of the facility 2.1. Gamma-ray sources To cover the wide dose rate range from few μGy/h to some tens of mGy/h, three 137Cs sources with very different activities were ordered from the chosen supplier (Eckert & Ziegler Nuclitec GmbH, Braunschweig, Germany). 137 Cs decays with a half-life of 30.05 years by β- emission to the ground state of 137Ba (emission energy 1175.63 keV, emission probability 5.64%) and via the 662 keV isomeric level of 137Ba (137 mBa) (emission energy 513.97 keV, emission probability 94.36%). The following decay of 137 mBa (half-life of 2.55 min) yields a gamma ray with an energy of 661.657 keV (emission probability 90%), an X-ray with an energy of about 32 keV (emission probability 9%) and Auger electrons (emission probability 1%). Therefore, from the decay of 137Cs we have gamma rays with energy of about 662 keV, with an emission probability of 84.92%, plus β- particles, X-rays and Auger electrons (Bureau International des Poids et Mesures, 2006). The three sources are extended sources, with the emitting volume constituted by a cylinder with a diameter of 4 mm and a height of 4 mm. The sources contain the radionuclides as a pellet of solid Cesium-ceramic doubly encapsulated in welded stainless steel. The temperature of 37 °C and the high level of humidity inside the cell culture incubator have no effect on the 137Cs sources, that are mechanically stable, insoluble in water and can be used in a wide range of temperature from −40 °C up to +80 °C . As of August 24, 2012, the total activities of the sources are 37 MBq, 740 MBq and 18.5 GBq, respectively. The sketch of a source is shown in Fig. 1. The β − particles and the Auger electrons are readily stopped by the 1.4 mm thick stainless steel layer encapsulating the source (ICRU, 1984). Considering the value of the mass attenuation coefficient, μ/ρ , for the stainless steel and for the energy of 32 keV, it can be seen that only about 0.07% of the X-rays with energy of 32 keV cross the stainless steel layer. The dose rate incident on the cells is, tehrefore, almost exclusively due to the 662 keV photons, while the contribution of the 32 keV X-rays is extremely low. In the following, in the section dedicated to the description of the GEANT4 simulations, the contributions to dose rate are quantified.

6.4 mm 4 mm 1.4 mm Cs-137 in form of ceramic

4 mm 11mm

Stainless steel

7mm

Fig. 1. Sketch of the 137Cs sources (manufacturer: Eckert & Ziegler Nuclitec GmbH).

G. Esposito et al. / Applied Radiation and Isotopes 115 (2016) 227–234

30 mm

229

1000 mm

50 mm

Lead shield

Sample

Wall Incubator

Lead cap

1247 mm

Sample

280 mm Lead irradiator Fig. 2. Sketch of the LIBIS irradiation facility.

Fig. 3. Photograph of the LIBIS facility with the shielding structure of the cell culture incubator. On the right a photograph of a steel copy of the source container.

2.2. The structure of the facility The irradiators are constituted by three special lead containers, each one housing one of the 137Cs sources. Thanks to their structure the containers act also as collimators. To carry an experiment, one of the irradiators is placed at the bottom of the incubator and its lead cap is remotely removed. Fig. 2 shows a sketch of the LIBIS irradiation facility (not to scale). It consists of a shielded large capacity cell culture incubator 2000 mm high, 970 mm wide and 840 mm deep, which shares the room with a Gammacell (Gammacell 40 Exactor, Nordion International Inc.) with a dose rate of about 0.75 Gy/min (as of 1st February 2016) and dose uniformity of about ±7% over a cylindrical volume with diameter of 260 mm and height of 100 mm. Operators safety is warranted: the incubator is shielded by two lateral lead plates 30 mm thick and one top lead plate 50 mm thick; thicknesses were evaluated with the Monte Carlo simulation program GEANT4 (http://geant4.web.cern. ch/geant4; see also (Agostinelli et al., 2003)). The other two sides of the incubator are shielded by the 1000 mm thick brick walls of the room. The shielding is such that during an experiment the contribution to gamma dose rate from the LIBIS sources in the room housing the facility, even in contact with the lead shields, is much lower than the environmental background radiation ( 0.3 μGy/h at ISS). Inside the incubator, the distance between the source and the cellular sample may vary between 280 mm and 1247 mm. The cellular sample is placed on a polyethylene support plate. Below 280 mm and above 1247 mm it is not possible to place the cellular sample due to the presence of the system handling the cap of the irradiator and of the top wall of the incubator. Fig. 3 shows, on the left, a picture of the facility, with the shielded incubator. On the right, a blow up of a steel copy of the source container and of the cap. This copy, without any source inside, is used for checking system electronics. 2.3. The GEANT4 simulations, the design of the irradiators and the incubator shielding We have used the Monte Carlo simulation code GEANT4 for all the stages of the design of the facility. This code has already been

proved appropriate for this kind of application (Gharbi et al., 2005; Kadri et al., 2006). The simulations were used to optimize the geometry and the characteristics of the facility. Sources, lead irradiators and lead shielding were simulated in all detail. The main features of the facility are: 1. percentage of primary photons (with 662 keV energy) at sample entrance greater than 80%; 2. dose rate uniformity such that the dose rate variations in the sample are less than or equal to 8%; 3. dose rate value in contact with the irradiators lower than 40 μGy/h ; 4. contribution to gamma dose rate given by the LIBIS sources about one order of magnitude lower than the environmental background radiation ( 0.3 μGy/h at ISS) in the area accessible to people of the room that houses the shielded incubator and the lead strongbox (where the irradiators are kept when not in use); moreover, dose rate values in contact with the top lead shield of the incubator (inaccessible to people) lower than 0.5 μGy/h , in order to avoid problems due to possible scattering of photons. The first point is a consequence of the fact that the sources are inside the incubator and no filter is used. This allows to have a beam as monochromatic as possible on the sample. The second point is the result of a compromise between a dose rate as uniform as possible on the sample and the necessity to have a sample as large as possible, for statistical accuracy. This compromise determines the irradiation area as a function of the distance of the sample from the source. The last two points concern radiation protection. 2.3.1. The sources and the lead containers The manufactured 137Cs sources (Fig. 1) and the emission of 662 keV photons were simulated in details in GEANT4. The cylindrical lead container was also simulated in all its parts, including the steel inserts and the lead cap. The polyethylene support plate and the polystyrene culture flasks were simulated at several distances between the source and the cells, that in turn

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were simulated as a 6 μm thick parallelepipeds filled with water. The dosimeters were simulated as parallelepipeds or cylinders filled with water. Finally the incubator and its lead shields were also simulated. The dose rate dD/dt at the cell or dosimeter position was calculated in two ways: I) The energy ϵ of the photons at the entrance of the area S of the cellular sample or dosimeter (obtained from the GEANT4 output) was used to compute the dose rate in μGy/h in that area with the following relationship:

⎛ ⎞ A(Bq) × Pem dD ⎜ μGy ⎟ 1 = 0.576 × ⎜ dt ⎝ h ⎟⎠ Nrun S(cm2)

Ns

∑ ϵi(MeV) × i=1

i ⎛ 2⎞ μen ⎜ cm ⎟ ⎜ ρ ⎝ g ⎟⎠

(1)

i where μen /ρ is the mass energy-absorption coefficient of a photon of energy ϵi in liquid water, A is the source activity, Pem is the emission probability of the photons, Nrun is the number of events (i.e. of emitted photons) in a GEANT4 run, Ns is the number of photons incident on S, and 0.576 is the conversion factor from MeV·g−1·s−1 to μGy  h  1. II) The dose rate was calculated from the total energy E deposited inside a generic volume of the cellular sample or dosimeter with mass M by the secondary electrons produced by Compton and photoelectric effects during a run of Nrun events, with the following relationship:

A(Bq) × Pem dD ⎛ μGy ⎞ E(MeV) × ⎜ ⎟ = 0.576 dt ⎝ h ⎠ Nrun M (g)

(2)

These two methods were compared for several test cases and a good agreement of results was obtained. Since the second method is more time consuming, the first method was employed for all the simulations. The number of events Nrun was chosen in order to obtain a statistical standard deviation lower than 5%. The standard deviation depends on the number Ns of photons incident on the sample, which in turn decreases as the distance between the source and the sample increases. Thus, depending on this distance, the number of generated events varied between 2  107 and 1  109. In a simulation that took into account the emission probability from the source of X-rays with energy of 32 keV, we checked whether their contribution to the dose rate on the sample was negligible, and we found that it was always lower than 0.05 μGy/h , even with the strongest source. Therefore in the following simulations only the gamma rays with energy of 662 keV were considered. On the basis of the simulation results three lead containers

were constructed, consisting of a lead holder and of a lead cap; Fig. 4 shows the different parts in detail. Each container houses one of the three 137Cs sources; Table 1 gives the dose rates for the three sources at the minimum and maximum distance. By placing the sample at intermediate distances we have irradiations with dose rates ranging between the minimum and the maximum of any given source; since the position of the polyethylene support plate can vary in 5 cm steps, an almost continuous variation of the dose rate can be achieved. The conditions inside the incubator do not affect the lead containers, that with high humidity only develop a harmless protective layer of PbCO3. Finally, the highest dose rate in contact with the irradiator with the strongest source is 31 μGy/h , fulfilling the third feature. 2.3.2. Dose rate uniformity For the experiments the polystyrene flasks containing the cell culture are placed on a 2 mm thick polyethylene plate inside the incubator, at the desired distance from the source. To study the sample irradiation, simulations were performed at four distances d between the source and the sample, i.e., 280 mm (the smallest possible distance), 550 mm, 1100 mm, and 1247 mm (the greatest possible distance). In each simulation one irradiator is at the bottom of the incubator with the cap removed. For each distance d the dose rates on the sample were calculated at different distances s from the center of the sample, placed on the vertical above the source. Fig. 5 shows the percentage ratios between the dose rate at distance s, Ḋ (s ), and the dose rate at the center, Ḋ (s = 0), as a function of the distance s. Dose rate variations in the irradiated area of less than or equal to 8%, are achieved up to distances s equal to about 60 mm for d = 280 mm , 135 mm for d = 550 mm , 250 mm for d = 1110 mm , and 280 mm for d = 1247 mm . The steep decrease of the percentage ratios for greater values of s is due to the shielding effect of the lead container/collimator. Table 1, in the last two columns, summarizes the data for the maximum and the minimum distances. The simulations also showed that the percentage of primary photons evaluated on the sample was always greater than 81%. 2.3.3. The lead shields and the strongbox Simulations were performed in order to evaluate the thicknesses of the incubator lead shields necessary to achieve the dose rate limits indicated in the fourth point of the list of features. The necessary values were found to be: 30 mm for the lateral shields of Steel insert

Lead cap 20 mm

Lead cap

22 mm 30 mm

63.5 mm

45 mm 10 mm

Steel insert 14 mm

Lead holder

Source housing 64 mm 129 mm

31 mm 6.8 mm

Lead holder

168 mm Fig. 4. Design of the lead container/collimator using the information obtained from the GEANT4 output.

G. Esposito et al. / Applied Radiation and Isotopes 115 (2016) 227–234

Table 1 Dose rate and dose rate uniformity values for the maximum and minimum distances d between the source and the sample for the three sources with the different activities A (at August 24, 2012); s is the distance of the considered sample point from the center of the polyethylene support plate supporting the sample. The second column from the right indicates the distance below which the dose rate uniformity of 92% is achieved. d(mm)

Ḋ (s = 0)

( ) μGy h

A¼ 37 MBq A ¼ 740 MBq 1247 ( dmax ) 2 40 280 ( dmin )

s(mm)

Ḋ (s) Ḋ (s = 0)

× 100

A ¼18.5 GBq

40

982

≤280

≥92

793

20,000

≤60

≥92

the incubator, 50 mm for the top shield, and 35 mm for the walls of the strongbox.

3. Experimental measurements 3.1. Gamma dosimetry In order to measure the dose, the dose rate and the dose uniformity at the position of the cell inside the incubator, an Area Monitor Probe (AMP-50), manufactured by the Rotem Industries Ltd. (Beer Sheva, Israel), and EBT3 GafChromic films, supplied by Tecnologie Avanzate (Turin, Italy), were used. The AMP-50 is equipped with a very sensitive Geiger-Müller tube, allowing to realize measurements in low dose rate fields (0.5 μGy/h − 40 mGy/h ); it has a sensitivity (137Cs) of 1.7 cps/μGy/h and an energy range from 70 keV up to 2 MeV; being waterproof, it can be used inside the cell culture incubator with a high level of humidity. Through a waterproof cable, the read-out of the detector

231

can be done on a laptop outside the shielded incubator. The AMP50 dosimeter was also used to measure the dose and dose rate close to, and in contact with, the irradiators. The EBT3 GafChromic films have a high spatial resolution (at least 25 μm ), allowing to measure with excellent reliability the dose uniformity on the cellular sample, and are nearly tissue equivalent. Widely used for photon dosimetry (Butson et al., 2010; Vandana et al., 2011; Villarreal-Barajas and Khan, 2014), they are self-developing films, made by laminating an active layer between two identical polyester layers. The films are designed for best performance in the dose range from 0.01 to 8 Gy. The EBT3 films used in the present work are sheets of 203.2 × 254.0 mm2 with a 0.028 mm thick active layer (sandwiched between two 0.125 mm thick polyester layers). They are yellow tinted plastic films with an active layer which colorizes upon exposure to ionizing radiation, involving a polymerization reaction that turns the film darker. The degree of coloration is measured in terms of the film net Optical Density (netOD) (Devic et al., 2004). The measurement of the dose rate outside the incubator shields and outside the lead strongbox containing the irradiators was done by the ISS qualified expert using an organic scintillator with a broad energy range from 23 keV up to 7 MeV., namely Automess unit 6150 ADB. It consists of a dose rate meter 6150AD5 (/H) and of the scintillator probe 6150 CE-b (76 mm diameter and 1.032 g/cm3 density) with high sensitivity (137Cs) of 300 cps/μGy/h . With this detector fast and accurate measurements can be done for very low dose rate values (starting from as low as 0.005 μGy/h ). 3.2. Comparison of experimental and simulation results At the design stage we used the source activities at the time of their construction (August 2012) to perform our computation,

Fig. 5. Predicted variations of dose rate on the flat surface housing the sample versus the distance s from the center of the sample for four distances d between the source and the sample. A horizontal line at the value 92% is plotted.

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Table 2 Experimental and calculated dose rates using the Area Monitor Probe (AMP-50) and GEANT4 simulations, where d is the distance between the source and the detector and s is the distance between the detector and the center of the polyethylene support plate supporting the detector. The data refer to the source with activity A = 18.5 GBq (at August 24, 2012). d (mm)

s (mm)

̇ (mGy/h) Dexp

̇ (mGy/h) Dsim

539

0 105

5.077 0.30 4.80 7 0.28

4.96 4.64

843

0 175

2.117 0.13 1.98 7 0.12

2.02 1.89

1045

0 205

1.36 7 0.08 1.30 7 0.08

1.33 1.23

1247

0 205

0.96 7 0.06 0.93 7 0.06

0.92 0.88

since radiation protection measures valid with the initial activities are valid also with somewhat decreased activities, as they are now. For the comparison with dose rate measurements we have obviously taken into account the activities at the time of the measurement, that have slightly decreased since their construction. The AMP-50 was used for dose rate measurements at sample positions inside the incubator and in contact with the irradiators. For the measurements at sample positions the probe head of the detector was placed on the support plate, contained inside the incubator, at four distances d between the source and the detector, i.e. 539 mm, 843 mm, 1045 mm and 1247 mm. For each distance d the dose rate was measured at the center of the support plane (i.e., on the vertical of the source) and at a distance s from the center as reported in Table 2. The measurement errors were calculated accounting for two contributions: the statistical error, equal to the square root of the number of counts, and the accuracy error of the detector. For comparison purposes the results from the GEANT4 simulations are reported in the last column of Table 2. There is good agreement between the measured and simulated dose rates. Measurements of the dose rate in contact with the irradiator whose source has the highest activity (18.5 GBq as of August 24, 2012) gave a maximum value of (30 72) μGy/h, again in agreement with the simulation value of 31 μGy/h . Measured with an organic scintillator, the dose rate values in contact with the lead strongbox containing the irradiators or in contact with the lead shields of the incubator were equal to the environmental background radiation ( 0.3 μGy/h ). The uniformity of the gamma dose throughout the sample area was measured using a EBT3 GafChromic film placed on the support plate at a distance d ¼539 mm from the source with activity of 18.5 GBq, with the center of the film was on the vertical of the source. A total dose of 1 Gy was chosen so to have an excellent performance of the detector. Since the dose rate at distance d ¼539 mm is about 5 mGy/h (Table 2) the film was exposed to the radiation for about 200 h, for a total dose of 1 Gy. The optical densities were measured for both unexposed and exposed films by sampling predefined 1.32 × 1.32 cm2 regions of interest (ROI) on the film image along the s axis passing through the center of the film. Fig. 6 shows the net optical density values, netOD, along the s axis normalized to the highest value of netOD and multiplied by 100. The figure shows the result of a GEANT4 simulation with the same distance d ¼539 mm and by sampling the same ROI. The dose rate values obtained by the simulation were also normalized to the highest value of dose rate and multiplied by 100. Again in this case the agreement is very good. Since only a small variation in response with dose rate has been reported for EBT films (Rink et al., 2007), the conversion of the netOD to radiation dose was

Fig. 6. Experimental and calculated relative beam profiles along the s axis, evaluated from the ratios between the measured net optical density values, netOD, with respect to the highest value of netOD (close circles), and the calculated dose rate values with respect to the highest dose rate value (open circles), respectively.

performed using a calibration curve obtained with the GAMMACELL 40 (Nordion) (dose rate of about 0.75 Gy/min). For the film considered above at d ¼539 mm we then obtained a dose value of 1.01 70.02 Gy, in agreement with the measure obtained with the AMP-50. 3.3. The irradiation procedure and the control system When not in use the irradiators are stored inside a strongbox shielded with lead (Fig. 7a). For the experiments, one irradiator is transported with a cart along a steel lane, and with an ad hoc arm that avoids direct contanct it is placed inside the incubator, only a few meters from the strongbox (Fig. 7b, c). The dose rate of about 31 μGy/h in contact with the irradiator housing the strongest source, considering the use of the special arm, can be considered safe for the operator. After placing the cell sample and closing the door, the cap is removed through a remote-controlled handling system (Fig. 7d). The correct movement of the cap is monitored through two systems inside the incubator: the AMP-50 gamma rays detector and an infrared camera. A meter box connected with the gamma detector can be read outside the incubator, discerning the dose rate value when the cap is inserted and the dose rate value when the cap is removed. At the same time an external monitor connected to the infrared camera allows to look at the images regarding the cap movement. All cables inside the incubator are isolated from moisture; they are brought outside the incubator and connected to monitoring systems through a small hole on a wall of the incubator without disturbing its operation. An Interlock System allows a safe operation: the incubator door cannot be opened unless the lead cap of the irradiator is completely inserted. At the end of the experiment the cap is restored by the remote system, and the irradiator is taken back inside the strongbox with the help of the cart.

4. Conclusions In this work a gamma irradiation facility for the exposure of cultured cells to low doses at a dose rates ranging from few μGy/h to some tens of mGy/h was described. The GEANT4 code was used for the design of the irradiators. The apparatus was simulated in order to optimize the geometry and the dosimetric characteristics of the irradiators, and to determine the size of the shielding thicknesses. Good agreement between simulation and

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233

Fig. 7. Irradiation procedure: a) strongbox where the irradiators are stored when not in used; b) cart used to move the irradiator towards the incubator; c) lane along which the irradiator is placed at the center of the incubator; d) remotely controlled handling system used to remove/restore the cap.

experimental results was obtained. The facility features a gamma beam with a percentage of primary photons (with 662 keV energy) at sample entrance greater than 81%. The dose rate uniformity is such that the dose rate variations in a rather extended irradiated area (from 133 cm2 up to 2642 cm2, depending on the source-tosample distance) are less than or equal to 8%. The LIBIS facility will make it possible to investigate different issues: i) the effects of low dose rate chronic exposures at different dose rates; ii) a comparison with the effects of low dose rates highLET (Linear Energy Transfer) exposures, as well as of sequential irradiation of the same sample with different radiation qualities, using the existing alpha particle irradiator at ISS (Esposito et al., 2006, 2009); iii) a comparison with the effects of acute exposures, obtained from the available Gammacell; iv) sequential irradiation studies (gamma-alpha or alpha-gamma) at various dose rates. All operations will be possible with the maximum level of safety for the operator. Considering the low dose rates reachable, the radiobiological research with LIBIS should be useful for studies in radiation protection and nuclear medicine.

Acknowledgements The construction of the facility was funded by Euratom (EC Contract FP7–249689, “DoReMi” NoE, http://www.doremi-noe.net/), through a dedicated task. The authors thank Dr. Claudia Dell'Omo, the ISS qualified expert, for the dose rate measurements in the room housing the facility, and Ms. Monica Brocco for the editing of the manuscript.

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