Internal Background

Background from radioactive contaminants intrinsic in the organic liquid scintillator is clas- sified as internal background. Organic liquid scintillator is likely to have very low levels of intrinsic radioactivity for two reasons:

1. Metal impurities (such as 238 U, 232

Th, 40

K) which typically exist in ionic form, are insoluble in non-polar organic solvents.

2. The liquid scintillator is synthesized from petroleum which has resided deep under- ground for millions of years. As a consequence, some activities (e.g. 14

C) will have decayed away and new cosmogenic activity is kept to a minimum.

Table 2.2 gives the allowed concentrations of several potential background elements in the scintillator, which were derived by Monte-Carlo simulations. The most important isotopes dangerous for BOREXINO are shortly reviewed here:

- 14

C is continuously produced in the atmosphere by cosmic rays via the reaction

14

N(n;p) 14

C. It decays by beta emission with an endpoint of 156 keV and a half life of 5730 a. In living organic matter the ratio14

C= 12

Cis about10 12

. The petrochemical origin of the carbon in the scintillator should guarantee sufficient age for the14

C to have decayed away. However, underground production through (,n) and (n,p) reactions on carbon results in a finite level of14

C that varies depending on the petroleum source. As carbon is part of the molecular structure of the scintillator itself, it cannot be removed by chemical purification methods. If the14

C level in the scintillator is too high, the only solution is to obtain scintillator from a different petroleum source. At a concentration of 14

C= 12

C = 10 18

there are about 70 decays per second in 300 tons of scintillator. The background in the neutrino window due to the poor energy resolution at low energies and pile-up events is still less than 1 event per day.

- 238

U decays to206

Pb by 8and 6decays. Assuming secular equilibrium in the decay chain, the tolerable level of238

U in the scintillator is< 10 16

g/g. The secular equilib- rium might be broken at the long-lived234

U, 230 Th, 226

Ra or 210

Pb isotopes. A special concern is 222

Rn, which is continuously produced by 238

U contained in the detector materials, and as a noble gas with a half life of 3.8 days can readily diffuse into the scin- tillator. Another problem is the long-lived radon daughter210

Pb, which will be plated on the surfaces that were exposed to air during the detector construction, and which decays with a half life of 22.3 a.

- 232

Th decays to208

Pb by 6and 4 decays. The tolerable concentration for 232

Th in the scintillator is about 10

16

g/g. The secular equilibrium may be broken at the long- lived228

Ra or228

Th isotope. The radon isotope220

Rn is not a problem, as it has only a short half life (55.6 s) and decays via short-lived daughters to the stable isotope208

2.4 Background

- Potassium is ubiquitous with a concentration of 2.4 % in the earth’s crust. Naturally occurring40

K has an isotopic abundance of1:210 4

with a half life of1:310 9

years; 89 % of its decays proceed by beta emission with an endpoint of 1.3 MeV and 11 % decay by electron capture with the emission of a 1.46 MeV gamma. The tolerable limit for BOREXINO is 10 14

g/g (Knat) in the scintillator. The potassium content in the fluor PPO as delivered was measured by NAA to be2
10

7

g/g [Gol97]. By water extraction of a concentrated solution (100 g/l PPO in PC) this could be reduced several orders of magnitude, so that the final potassium concentration in the CTF scintillator (1.5 g/l PPO in PC) was below the sensitivity limit of both the CTF (<2
10

11

g/g) and NAA (<4
10

12

g/g). The purity requirement is thus not directly confirmed, but it is expected to be met because of the high efficiency of the water extraction.

- 7

Be can be formed by proton and neutron reactions on 12

C from cosmic radiation. It decays by electron capture with a half life of 53 days, with a 10 %-branch emitting a 478 keV gamma. In equilibrium at the earth’s surface7

Be gammas are expected to give a count rate of 2700 per day and ton of scintillator [Vog96]. This rate can be reduced by bringing the scintillator underground directly after distillation, before the equilibrium rate of 7

Be is reached (after one day of surface exposure the expected gamma rate is 30per day and ton), and storing the scintillator underground for a sufficient period of time to allow the7

Be to decay, or by removing the7

Be through purification. At a small rate,7

Be will also be produced on site by cosmic ray muons (see paragraph 2.4.3). Though there is no clear signature for a single neutrino event, there are several methods to discriminate certain classes of background events:

- correlated events: the method consists of tagging delayed coincidences in the U and Th decay chains (214 Bi-214 Po, - with t 1=2 = 164s; 212 Bi-212 Po, - with t 1=2 =

Isotope Abundance T1=2[a] Concentration [g/g]

113 Cd 12.2 % 9:3
10 15 2:8
10 9 115 In 95.8 % 4:4
10 14 2:4
10 12 40 K 0.01 % 1:3
10 9 7:8
10 15 138 La 0.09 % 1:1
10 11 1:2
10 12 176 Lu 2.59 % 3:8
10 10 3:7
10 15 87 Rb 27.8 % 4:7
10 10 3:2
10 14 232 Th 100 % 1:4
10 10 2:4
10 16 238 U 99.3 % 4:5
10 9 1:0
10 17

Table 2.2: Trace element concentrations at an equal amount of 1 event/(100t day) to the back-

ground rate in BOREXINO in the energy range from 250 - 800 keV, after application of all cuts (correlated events,/discrimination, statistical subtraction).

300ns; 220

Rn-216

Po, -with t 1=2

=0:15s), which can be identified using a cut on the energy and the coincidence time.

- pulse-shape-discrimination: in organic scintillators, the pulse shape is different for and events, which can be used for identifying and rejecting internal background events. A detailed description of this method will be given in chap. 5.

- statistical subtraction: after tagging the delayed coincidences in the uran and tho- rium chain, also the activity of the corresponding parent isotope is known, if they are in equilibrium. This is a valid assumption only for short-lived, non-mobile isotopes. In the 238

U chain, this can be done back to the mobile222

Rn isotope (t1=2 =3:8d); in the 232 Th chain back to 228 Th (t1=2

= 1:9a). When the activity of the parent isotope is known, then the spectrum of all following decays can be subtracted. This subtraction cannot be done event by event, but only after a sufficient statistics of correlated events has been accumulated.