BGO and radiation hardness

BGO has very desirable characteristics for electromagnetic calorimeters:

• radiation hardness at megarad level,
• excellent $e/\gamma$ energy resolution of (0.3 - 1)%/$\sqrt{E(GeV)}$,
• high density of 7.1 gm/cm$^3$,
• short radiation length of 1.12 cm,
• large refractive index of 2.15,
• suitable scintillating properties with the fast decay time of about 300 ns and peak scintillation at about 480 nm, and
• non-hygroscopic nature.

Pure BGO crystals with silicon photodiodes were proven to be capable of detecting minimum-ionizing particles (MIPs) with a large S/N ratio [9]. In the same experiment the nuclear counter effect (NCE) was also clearly observed. NCE is the extra amount of charge produced in the photodiode by a charged particle directly hitting it, on the top of the charge produced by the scintillation light. The signal from MIPs is well separated from electronics noise and NCE signal.
The radiation hardness of undoped BGO crystals from two manufacturers, the Institute of Single Crystals, Kharkov, Ukraine and the Institute of Inorganic Chemistry, Novosibirsk, Russia, was measured [10]. BGO crystals from Novosibirsk are found to be quite radiation hard at least up to 10.5 Mrad equivalent dose with a maximum of 15 % damaging effect, while Kharkov crystals degrade by more than 40 % in terms of light output and transparency. It was concluded that BGO crystals from Novosivirsk can be used for the Belle detector.
Radiation damage tests were carried out using a $^{60}$Co source ($\sim$1000 C) at National Tsing University, Taiwan [11]. A total of 10 pieces of undoped trapezoid BGO crystals with an approximate dimension of 12 $\times$ 2 $\times$ 1 cm$^3$ from Novosibirsk were used in the tests. Fig. shows the experimental setup. Trapezoid BGO crystals were irradiated longitudinally (LBGO) or transversely (TBGO). Light outputs were measured using a 90 $\mu$C $^{137}$Cs source after irradiation and the ratio of signals from an irradiated crystal and the non-irradiated reference crystal was obtained. Fig. shows some of the results.

Figure: Top view of the experimental setup in the radiation source room. BGO crysals were 3 cm away from the center of a $^{60}$Co source of $\sim$ 1000 C.

Stable characteristics of the BGO crystals were observed under very high dose conditions. The light output decreases by $\sim$30 % after receiving $\sim$10 Mrad dose. The crystals may have suffered some permanent damage but can recover to 90 % of their original light output in a period of a day [12]. Furthermore, after a low dose irradiation of 115 krad the light output recovers to 100 % of the original value within hours, as shown in Fig. . After receiving the BGO crystals used in the final assembly, the scintillation light yields of all the crystals were checked by a radioactive source. The percentage spread of the distribution is only 6 % [13]. The radiation hardness check was done by using sample crystals, two per ingot, and some randomly selected crystals of the final assembly. Their performance characteristics are quite different from those of crystals produced in a small quantity. The light yield drops about 25 - 50 % after receiving 1 krad dose and remains stable afterwards, checked up to 10 Mrad. The irradiation rate to receive 1 krad dose does not make much difference since the recovery is slow, in days or weeks. This characteristics is good for a stable operation in the Belle environment.

Figure: Results of the first week irradiation. The horizontal scale is in equivalent doses and the vertical scale is the light yield normalized to the original value without irradiation: (a) and (b) correspond to the crystals irradiated longitudinally, and (c) and (d) to those irradiated transversely.

Figure: BGO test crystal 1 received a dose of 115 krad in one hour at its front surface. The time dependence of the relative output is shown before and after the irradiation.

Radiation damage tests on optical reflector [14] and epoxy-based optical glue [15] which are used in the BGO detector assembly were performed. The reflectance of optical white fluorocarbon reflector, Goretex [16], was monitored to see any effect of radiation damage. No radiation damage was observed for the maximum equivalent dose of 8.6 Mrad. The epoxy-based optical glue was found later to be inadequate to bond the photodiode surface on a BGO crystal. An RTV type of glue, KE45T, was proved to be good. Its radiation hardness was checked acceptable up to 10 Mrad maximum equivalent dose.
Photodiodes are good for a radiation dose of about 50 krad and the least radiation-hard element. They are, however, protected behind the BGO crystals, the container, and the accelerator magnets. In case they are radiation damaged, they can be replaced relatively easily.