
^{3}H (1987TI07)(Not illustrated) Ground State:
μ = 2.978960 ± 0.000001 nm, M  A = 14.94991 ± 0.00003 MeV.
The rms charge and magnetic radii for ^{3}H determined from electron scattering (see reaction 8 (a)) are γ^{c}_{rms} = 1.63 ± 0.03 fm and γ^{m}_{rms} = 1.72 ± 0.06 fm. In general twobody force calculations give values of γ^{c}_{rms} which are ≈ 10% too large (1986GIZS). This discrepancy has not yet been fully resolved by the addition of threebody forces although there are calculations (1986IS06) which, when extrapolated to give the correct triton binding energies, are in resonable agreement with γ^{c}_{rms} (exp). (However, the form factor problem remains (1986GIZS)). See also (1985FR12, 1986SAZG) which examine the way in which the triton bound state observables scale with binding energy. The binding energy of ^{3}H is 8.481855 ± 0.000013 MeV (1985WA02). Many calculations have been done to predict the binding energy of ^{3}H and ^{3}He (see the reviews of (1984FR16), 1986FRZU, 1986GIZS) and references given below. It is observed in (1986GIZS) that twobody force calculations with realistic forces underbind ^{3}H by ≈ 1 MeV whereas calculations with threebody forces give binding energies too large by ≈ 0.5 MeV, although it is pointed out in (1986SA2A) that threebody force calculations can give correct biding energies if the cutoff mass is taken to be 700 MeV. Charge and magnetic form factors for ^{3}H have been determined from electron scattering experiments (see reaction 8 (a)). Measurements for q^{2} from 23 to 31 fm^{2} are reported in (1985JU01). The available data indicate that the magnetic form factor is similar to that for ^{3}H which has a diffraction minimum at a higher value of q than predicted by impulse approximation calculations. The isobar model of (1983ST11) with mesonexchange currents satisfactorily accounts for the differences. (See also (1986SA07, 1986SA08).) Calculations of the charge form factor with twobody potentials are in serious disagreement with experiment for ^{3}He in that the theoretical momentum transfer at the first minimum is too high, and the height of the second maximum is too low. (See also reaction 8 (a).) However it is pointed out in (1987PL2A, 1987ST09) that the ^{3}H charge form factor is well described by theory if the correct pseudoscalar vector coupling for the pionnucleon vertex is used in the calculation of the exchange current contributions. For other recent calculations see (1985BO44, 1985MA24, 1986CI05, 1986FR15, 1986HA12, 1986KI17, 1986LI09, 1986SA07, 1986SA08). The addition of a threebody force increases the calculated value of the charge form factor in the region of the second maximum by 50%, but a factor of three is needed (1986GIZS). The very recent work of (1987BE30) reporting on measurements of ^{3}H and ^{3}He isoscalar and isovector form factors also reviews the extent of agreement between current theories and experiment. The review of (1986FRZU) notes that calculational techniques for the trinucleon system have progressed to the point where critical examinations of threenucleon forces, relativistic effects of nucleon motion, and explicit nonnuclear degrees of freedom such as pions, isobars, quarks etc. can be made with some confidence. A great deal of work has been done to explore threebody force effects. See for example the conference proceedings (1984FA2A, 1986BE2A) and summaries (1986CA2A, 1986CH2C, 1986FRZU, 1986GIZS, 1986TO2A) and the review (1984FR16). See also recent calculations of (1976HA40, 1977HE22, 1979OS08, 1979YA03, 1981CA14, 1981CO2B, 1981LO08, 1982CO04, 1982DA06, 1982DA19, 1982DA20, 1982DA25, 1982GL05, 1982MU13, 1982ZA01, 1983CA10, 1983CO17, 1983ME14, 1983MU06, 1983WI05, 1984CO18, 1984IS04, 1984UE01, 1984WI12, 1985CH17, 1985CH22, 1985RO02, 1986CH13, 1986SA08, 1987NA02, 1987ST09). Relativistic effects in the trinucleon bound state are considered in (1977BA25, 1979HA36, 1980MA22, 1981GA15, 1981KO02, 1986GL01). The effects of including tensor forces in threebody calculations are discussed in (1986OS04, 1986OS07). The quark structure of the trinucleon and the effect of quark clusters on its ground state properties have been studied by (1983BE08, 1983NE08, 1983NE12, 1984GR18, 1984SU03, 1985FA01, 1985KO02, 1985MA24, 1985MA30, 1985MA67, 1985VE15, 1986AB02, 1986BH05, 1986CH22, 1986GU12, 1986KI17, 1986OE02, 1986SA02). Trinucleon asymptotic normalization constants have been calculated by (1976LE01, 1979GI2B, 1979KI03, 1980HA03, 1980SA23, 1982BO22, 1982FR07, 1984FR16, 1986IS01). Binding energy calculations in addition to those mentioned above have been carried out by ( 1975HE11, 1976DZ01, 1976SH16, 1977EL05, 1977GL07, 1977HE16, 1977NU2A, 1978BI02, 1978CH21, 1978EF01, 1978GI10, 1978KI14, 1979HA27, 1979OS02, 1979SA15, 1980BA21, 1980GO10, 1980MI19, 1980PA12, 1981HA41, 1982BA58, 1982DA24, 1983HA19, 1983OR06, 1984GI01, 1984JI05, 1984OR02, 1985AB12, 1985EF01, 1985OR03, 1985OR05, 1985OR06, 1985OR08, 1985RU04, 1986BE25, 1986HA10, 1986HA36, 1986KE07, 1986NA10, 1986PE08, 1987TO04). Theoretical papers not mentioned elsewhere are (1975AF01, 1975GO04, 1975MC17, 1977AF02, 1977BA20, 1977BE61, 1977BL11, 1977EM01, 1977SO05, 1978HY01, 1978ST28, 1979BE09, 1979DE13, 1979PE06, 1980ZN01, 1981KO37, 1981LE22, 1981PA05, 1981SA27, 1982AT01, 1983GR24, 1983ME06, 1985BO17).
Early measurements of the halflife are reviewed in (1975FI08). A recent evaluation of available experimental data was carried out by (1984HO2A). The recommended value is τ_{1/2} is 12.3 ± 0.1 years. The standard deviation on the recommended value is based on the disagreement between the evaluated measurements. A very recent measurement reported in (1987SI01) gave τ_{1/2} = 12.32 ± 0.03 years. The Qvalue adopted by (1985WA02) is 18.594 ± 0.008 keV. A recent measurement by (1985SI07) with a tritium implanted Si(Li) detector gave 18.577 ± 0.007 KeV for the ^{3}H end point energy. See also the related massdifference measurements of (1981SM02, 1984LI24, 1984NI16, 1985LI02). Using the evaluated data for the half life and endpoint betadecay energy, (1978RA2A) obtained a value for the GamowTeller matrix element, < σ · τ > = √ 3(square root of 3)(0.975 ± 0.007). This was based on a value of the ratio of axialvector to polarvector coupling constants G_{A}/G_{V'} = 1.237 ± 0.008. On the basis of more recent data, (1982BA20) has suggested using G_{A}/G_{V'} = 1.259 ± 0.009. This results in a value for the GamowTeller matrix element of < σ · τ > = √ 3(square root of 3)(0.958 ± 0.008) which (1982BA20) compares with calculated values in various approximations. The value calculated using π^{} and ρ^{} exchange with point couplings agrees with this modified value. calculation of < σ · τ > including axial meson exchange current effects (1984CI01) gave agreement with experiment. An experiment to measure < σ · τ > in a modelindependent way is discussed in (1985BUZZ). The effect of the atomic and molecular environment on the value of < σ · τ > deduced from experiment was studied by (1983BU13) and found to be significant, and could imply a higher probability of finding delta isobars in the triton. An analysis by (1984BO03) of experimental results which included ^{3}H beta decay placed limits on unusual coupling constants. Measurements of the ^{3}H beta spectrum to determine the antineutrino mass were carried out by (1981SI18, 1983DE47) who determined m_{ν} < 65 eV and m_{ν} < 50 eV, respectively. The experiment of (1980KO2A, 1980LU2A, 1981LU07) on the ^{3}H beta spectrum in the valine molecule indicated a finite antineutrino mass, 14 < m_{ν} < 46 eV. The effects of molecular structure on the ^{3}H beta spectrum shape were studied by (1982KA1X, 1983KA33) who determined that the lower limit of 14 eV should be replaced by a higher value. See also (1985KA21). However, the experiments of (1980KO2A, 1980LU2A, 1981LU07) were reevaluated by (1984SI2B) and (1985BE01) who found that there is no conclusive evidence for m_{ν} > 0. More recent studies by (1985BO34, 1985BO53) with improved apparatus and techniques found m_{ν} > 20 eV. Very recently an analysis (1987BO07) of ^{3}H decay in valine gave a neutrino mass of 30.3^{+2}_{8} eV while measurements of free molecular tritium decay reported in (1987WI07) gave an upper limit of 27 eV for m_{ν}. In other work bearing on the antineutrino mass measurements it was found that the effects of Coulomb corrections (1983WU01) and radiativespectrum corrections (1983RE13) could prove important in the determination of the neutrino mass from the ^{3}H beta spectrum. The effects of atomic final state interactions on the neutrino massdetermination problem were studied by (1983WI02) and found to be negligible. Twostep processes were examined by (1984ST09) and found to be unimportant. See also the recent work of (1986EM01, 1986LI08, 1987DR03). An experiment utilizing free atomic and molecular tritium is described in (1985KNZX). The beta spectrum of the tritium molecule is computed by (1985FA05), and it is shown that molecular effects are crucial in determinations of the neutrino mass. See also the recent work of (1986AR07, 1986AR18, 1986AR19, 1987SZ01). Electron energy losses were studied in (1986GE03) for the effect on neutrino mass determinations. A method for determining the neutrino mass by means of the photon spectrum from radiative beta capture in ^{3}H is discussed by (1985PA25) which also contains a review of neutrino mass determinations. A study of the effects of a possible neutrino mass on ^{3}H longitudinal polarization etc. was reported in (1986KE08). The possibility of an influence by intense electromagnetic waves on the beta decay of polarized (1983TE04, 1983TE06, 1984TE02, 1984TE03) or unpolarized (1983TE03) ^{3}H has been studied. An experimental test of laser enhancement of ^{3}H beta decay (1985BE26) found no effect. The possibility of observing recoilless resonant neutrino absorption in ^{3}H beta decay was pointed out (1983KE07). Other calculations are found in (1975BE49).
Measurements of the cross section cited in the previous review (1975FI08) included a measurement at 0.01 eV, six measurements at thermal energies, one at 2.4 MeV and one at 14.4 MeV. No information on gamma angular distributions had been obtained. More recent measurements made at thermal energies (1973IS08, 1979ALZL, 1982JU01) are listed in Table 3.1 (in PDF or PS). As noted in (1975FI08) the calculation of (1973HA30) which included meson exchange currents gave a thermal neutron capture cross section of 0.52 ± 0.05 mb in good agreement with experiment. See also (1981SH25) for a review of experimental and theoretical results for this reaction. A recent calculation (1983TO12) investigated the role of meson currents along with the use of wave functions obtained from the Faddeev equations using realistic NN forces as well as the effects of threenucleon forces. The capture of thermal neutrons by deuterons proceeds predominantly via a magnetic dipole transition into the S' state of mixed spatial symmetry in ^{3}H (1975FI08). The effects of S' and D state admixtures in ^{3}H are discussed by (1973HA30, 1981SH25, 1983TO12). Measurements of doubly radiative thermal neutron capture have been made by (1977MC05) who found an upper limit σ(2γ) = 8 ± 15 μb for 700 keV < E_{γ} < 5550 keV. See also (1979WU05). Calculations with a singleparticle direct capture model (1976LE27) gave σ(2γ) = 21 nb, while a detailed threeparticle calculation (1977MC05) gave σ(2γ) = 26 nb. An analysis (1974MC06) of parity nonconserving amplitudes in ^{2}H(n, γ)^{3}H indicates that gamma circular polarization and asymmetry depends on isoscalar and isovector parity nonconserving interactions respectively. See also (1986DE24, 1986DU14). The asymmetry of the photons from polarized thermal neutron capture was measured by (1984AV2A) to be (7.8 ± 3.4) × 10^{6}. A very recent measurement is reported in (1986AV04). Values of the weak coupling constants from the implied parity violations are discussed by (1985DO02, 1985MI10). At higher neutron energies, measurements of differential cross sections σ(θ) and analyzing powers A(θ) have been made by (1986MI17) and are listed in Table 3.2 (in PDF or PS). Absolute values for the angleintegrated cross section after detailed balancing are in good agreement with the inverse reaction (see section on ^{3}H(γ, n)). The foreaft asymmetry, defined by
where the fore and aft angles are the zeros of P_{2}(θ) (approximately 55° and 125°), indicates an anomalously large E2 strength in the ^{2}H(n, γ) reaction. By using the (p, γ) asymmetries from (1984KI06), the ratio a_{s}(n, γ)/a_{s}(p, γ) is determined to be ≈ 0.5 in disagreement with the factor of 0.2 expected from effective charge arguments. This result is consistent with the result obtained from the inverse reaction measurements of (1981SK02).
Measurements of total cross sections up to 270 GeV, differential scattering cross sections up to 152 MeV, and polarizations and analyzing powers upt to 35 MeV have been reviewed in (1975FI08). More recently total crosssection measurements were made at 4.2 MeV by (1975CA30) and from 0.07 to 20 MeV by (1980PH01). References for differential scattering, polarization and analyzing power measurements reported since (1975FI08) are listed in Tables 3.3 (in PDF or PS) and 3.4 (in PDF or PS). The values cited in the neutron crosssection compilation of (1981MUZQ) for the two scattering lengths (^{2S+1}a) for low energy nd scattering (S is the channel spin) are ^{4}a = 6.34 ± 0.02 fm and ^{2}a = 0.65 ± 0.03 fm. (Note however the measurement of (1975CA30) which is in disagreement.) These values are consistent with theoretical calculations cited in (1975FI08) and are well reproduced by the calculation of (1982PA21) based on a formulation using zeroenergy Faddeevtype equations and the swave interaction model of Malfliet and Tjon (1969MA2A). Other calculations of ^{4}a and ^{2}a were made by (1975BA2B, 1975WH02, 1978AL22, 1980HA40, 1981BE18, 1983PE18, and 1983ZA06, 1986PE08). The possible effect of the threenucleon force on ^{2}a has been studied by (1984DE20). For discussions of the correlation between the nd scattering lengths and the triton binding energy, see the ^{3}H General section of this compilation. Phaseshift analyses for cross sections and polarization data have been carried out at 2.45 MeV (1974BO04 and 1978BO28), and at 15  50 MeV (1986KL04). Theoretical predictions of nd phase shifts for energies below 45 MeV were made by (1974LA16, 1975AL07, 1975WH02, 1978FU01, 1978ST06, 1980HA40, 1982DU20, 1983KU08, 1987TO04). A phase shift calculation near the deuteronbreakup threshold was done by (1983PO04). Calculations of elastic scattering cross sections for E_{n} below 50 MeV have been made by (1975AL07, 1975MO17, 1975ST11, 1976BE19, 1977OR01, 1978AL22, 1978ST06, 1980HA40, 1981ZA06, 1982DU20, 1983KU08, 1986HA36). Theoretical predictions of polarizations and analyzing powers in nd scattering below 50 MeV have been made by (1974DO2A, 1975ST11, 1976BE19, 1978ST06, and 1981ZA06, 1986HA36). See also (1981ZH02, 1983MO24). A calculation of differential cross section and deuteron tensor analyzing power for incident energies 0.3 to 1 GeV taking into account the contributions of tribaryon resonances has been done by (1981KO09). Variational methods with separable potentials were used by (1974BR01, 1979ST17) for calculating offshell amplitudes for ^{2}H(n, n). Most of the aforementioned calculations of threenucleon observables involved solution of some version of the Faddeev equtions or application of various approximation methods in the framework of the Faddeev equations. (See the General discussion on ^{3}H for more details.)
Measurements and summaries (S) of deuteron breakup by neutrons published from 1974 to the present are listed in Table 3.5 (in PDF or PS). Earlier work is reviewed in (1975FI08). In a threeparticle reaction, five kinematic variables must be measured to make the experiment kinematically complete (e.g. E_{ni}, θ_{n1}, E_{n2}, θ_{n2}, E_{p}). Table 3.5 (in PDF or PS) indicates the particles detected and the coincidence requirements. Detector angles are not given explicitly, but the type of geometry, the main emphasis of each experiment and the region of phase space explored is indicated. Some of the experimental difficulties associated with measurements of the breakup process are discussed in (1974TH2A). A system designed for detecting two neutrons is presented, and various ways to reduce the background are discussed. Measuremants of neutron analyzing power in the n + d breakup reaction have been made at incident neutron energies of 14.3 and 29.6 MeV by (1978FI2A) (see Table 3.5 (in PDF or PS)). No others have been reported. References to several early reviews of experimental and theoretical wrok on the threebody breakup reactions are given in (1975FI08) along with a brief discussion of the general features of the cross section and particle spectra. For more recent reviews see (1976SL2A, 1978KU13, 1978SL2A). The total cross section for n + d breakup measured by (1975PA21) and others (see reviews (1976SL2A, 1978KU13)) increases almost linearly from zero at threshold (3.34 MeV) to a maximum of ≈ 180 mb at 12.2 MeV and then decreases slowly to ≈ 100 mb at 47 MeV. The peaks or enhancements in the proton energy distribution observed in nd breakup are associated (1975FI08) with the np and nn final state interactions (FSI) and the nn and np quasifree scattering (QFS). See (1975KU25, 1976SL2A, 1978KU13) for extensive discussions of these features including the kinematics of the processes. Calculations based on Faddeev formalism and simple NN interactions correctly predict both shapes and magnitudes of the breakup spectra (1976SL2A, 1978KU13). Experimental and theoretical investigations of the QFS portion of the proton spectra are reviewed and discussed in (1976SL2A and 1978KU13). See also (1975BO15, 1977FU05, 1978CA2A, 1979SO02, 1980GU11, 1980VO06). Many measurements of the ^{2}H(n, p)^{2}n reaction have been carried out for the purpose of extracting the nn scattering length a_{nn} and the effective range γ_{nn}. Early work is reviewed in (1975FI08). For more recent work see references of Table 3.5 (in PDF or PS) and the reviews mentioned above. An exhaustive review of experimental and theoretical methods of determining a_{nn} including discussions of possible chargesymmetry breaking and violations of charge independence implied by the results is given in (1975KU25). The review of (1976SL2A) discusses all aspects of the n + d breakup reaction with emphasis on experimental data and the relative accuracy of various methods for extracting a_{nn}. It is apparent in these reviews that the values quoted vary slightly depending on the classes and subclasses of experiments that are included in the averages and the type of analysis. An average value of a_{nn} = 16.3 ± 0.6 fm is obtained (1976SL2A) from kinematically complete ^{2}H(n, p)^{2}n experiments using Faddeev theory (see also 1978SL2A). For the nn effective range γ_{nn}, the value obtained by (1985SL2A) is γ_{nn} = 2.76 ± 0.11 fm. It is concluded (1975KU25) that the difference between a_{nn} and the neutron proton scattering length a_{np} provides clearcut evidence for charge independence violation, but a discrepancy between a_{nn} and the protonproton scattering length a_{pp} (after Coulomb correction) may not necessarily imply breaking of charge symmetry because of the dependence of the extracted value of a_{pp} on offenergyshell deuteron breakup by protons as well as neutrons. The effect of threenucleon forces (i.e. forces which depend in an irreducible way on the simultaneous coordinates of three nucleons when only nucleon degrees of freedom are taken into account) on the nd breakup process is investigated by (1984ME03), and it is determined that such a force could produce noticeable effects. In (1982SL2A) it is suggested that the difference between the accepted value of a_{nn} (16.3 ± 0.6 fm) and the value obtained from the ^{2}H(π^{}, p)^{2}n reaction (18.6 ± 0.48 fm) can be explained by a threebody force and that the effects of this force are different for neutron pickup and proton knockon processes in the ^{2}H(n, p)2n reaction. This suggestion is examined in the review of trinucleon properties by (1984FR16), and it is concluded here that evidence for significant threenucleon force effects is largely circumstantial, but nontrivial. See also the reviews of (1986GIZS, 1986TO2A). It is shown in work reported in (1984SL02) that corrections arising from the magnetic dipole interactions are relevant to the discrepancy in scattering parameters deduced from defferent reactions. It is suggested (1982SV01, 1984FR16) that the triple differential cross sections for ^{2}H(n, p)2n could be used to test for the presence of tensor force effects. See also the calculations of (1977ST16, 1979ST05) which show significant differences in the predictions of breakup reaction observables by different potential models.
Many new experimental and theoretical studies of the ^{2}H(p, π^{+})^{3}H reaction have been made in the past few years. Measurements and summaries of differential cross sections and analyzing powers obtained with polarized proton beams published since the previous compilation (1975FI08) are listed in Table 3.6 (in PDF or PS). A survey of experimental and theoretical work on (p, π^{+}) reactions, including reaction 5, can be found in (1981FE2A). See also the reviews of (1979HO2A, 1979ME2A). The differential cross section for the reaction is forward peaked. For example at 5°(lab) (1977AS06) has reported σ(θ) = 47, 28, 8 μb/sr at E_{p} = 410, 605 and 809 MeV respectively, while at back angles 110° < θ_{cm} < 160° the cross section is nearly flat and within the range 0.9 to 1.25 μb/sr at E_{p} = 425, 450, 475, 500 MeV (1984AB2A). Analyzing power angular distributions are characterized (1984LO08, 1982LO17) by a gradual change in shape from a negtive maximum near 100° at 277 MeV to a large positive maximum near 90° at 500 MeV (1981CA08). Recent data at 650 and 800 MeV (1984KI2D) show unexpected energy and backangle structure in both differential cross section and analyzing power at 800 MeV, and the authors speculate that the anomaly is related to the deltadelta component in the deuterion ground state. Also, large structure were seen in the 0.6  1.5 GeV back angle data of (1985BE46), who suggest possible baryonic delta excitations in the intermediate state. Experimental and theoretical interest in the (p, π^{+}) reaction on light nuclei was originally stimulated by the prospect that the reaction would provide a probe of nuclear structure at high momentum transfer, but problems with understanding the reaction mechanism have proved to be a barrier to this objective. Various theoretical approaches to the problem (DWBA singlenucleon mechanisms, other singlenucleon mechanisms, and twonucleon models) are reviewed in (1979HO2A, 1979ME2A, 1981FE2A). The theoretical situation for ^{2}H(p, π^{+})^{3}H is unsatisfactory. No detailed calculations have been sucessful in describing both differential cross sections and analyzing powers (1984LO08). Work in development since the previous review includes the microscopic twonucleon model calculations of (1982IQ2A) and (1982DI2A) and the isobardoorway model work of (1984KE02). Various wave function effects within the coupled channels deltaisobar model were investigated by (1982SA25). A recent calculation of all helicity amplitudes for ^{2}H(p, π^{+}) in the GeV region with the relativistic model is reported in (1986LO02). For other recent theoretical work see (1977GI06, 1978IS06, 1979GR03, 1979GR12, 1979GR19, 1979LA02, 1981BL12, 1981KO04).
Only one measurement for this reaction has been reported since the pervious compilation (1975FI08). Measurements of neutral and charged pion photoproduction in ^{3}H and ^{3}He by bremsstrahlung photons with E_{max} = 500 MeV were reported in (1984BE08). See also reactions 6 and 7 in ^{3}He for related information.
Only a few measurements of the photodisintegration of ^{3}H had been done prior to 1974, and they are listed in the previous compilation (1975FI08). More recently, both the twobody reaction (a) and the threebody reaction (b) photodisintegration cross sections were measured simultaneously from threshold to ≈ 25  32 MeV (1980FA03, 1981FA03). Monoenergetic photons were used and neutrons were detected. (See also 1981FA03) for a thorough review of experimental work.) For reaction (a) the cross section rises sharply from threshold to maximum of ≈ 0.9 mb at 12 MeV, then decreases only slightly to 0.8 mb at 19 MeV. For reaction (b) the cross section rises sharply from threshold to a maximum of ≈ 0.9 mb at 14 MeV and then falls smoothly to ≈ 0.4 mb at 26 MeV. The experiments of (1981FA03) included measurements of the ^{3}He(γ, n)2p reaction by the same techniques used for reactions (a) and (b). Detailed comparisons of the three reactions measured and of previous ^{3}He(γ, p)^{2}H data (1973TI05, 1975FI08) were made. The results were (a) the twobody breakup cross sections for ^{3}H and ^{3}He have nearly the same shape but the^{3}He cross section is lower in magnitude, (b) the threebody breakup cross section for ^{3}He is higher in magnitude, broader in the peak region, and rises less sharply from threshold than for ^{3}H, and (c) the differences between the cross sections for the breakup modes largely compensate in their sum so that the total photon absorption cross section is nearly the same for ^{3}H and ^{3}He. The integrated cross sections and their first and second moments (1981FA03) are listed in Table 3.7 (in PDF or PS). Measurements of differential cross sections for reaction (a) at angles from 45°  135° at photon energies of 6.7, 7.6, and 9.0 MeV, and at 90° over the energy range from 18  31 MeV were listed in (1975FI08). More recently (1981SK02) measured the cross section at 55°, 90°, and 125° over the energy range from 15  36 MeV by detecting the deuterons. From these data and published data for ^{3}He(γ, p) (see section on ^{3}He(γ, p)) the ^{3}H foreaft asymmetry (see section on ^{2}H(n, γ)) is found not to be 1/5 that for ^{3}He as predicted by simple effective charge arguments, but (although negative) it is only about half the magnitude of that of ^{3}He(γ, p), and has approximately the same energy dependence. This is consistent with the results obtained from the inverse reaction (1986MI17) (see section on ^{2}H(n, γ)). For reaction (b) measurements of the differential cross section for angles 45°  135° made at a photon energy of 10.8 MeV, and at 90° for photon energies 18  31 MeV are listed in (1975FI08), No new measurements of the ^{3}H(γ, p)2n differential cross section have been reported. Many theoretical treatments of the trinucleon photoeffect do not distinguish between ^{3}H and ^{3}He. Thus the section on ^{3}He should be consulted in addition to the work listed here. The previous compilation (1975FI08) includes references to a number of calculations of the excitation function for ^{3}H(γ, n) as well as calculations of the integrated and bremsstrahlungweighted cross sections and discussions of sum rules for A = 3 photodisintegrations. See also the review of (1977CI2A) and see (1981FA03) which contains an extensive summary of theoretical work for trinucleon photodisintergration. A calculation by (1975GI01) using a separablepotential Faddeev mode explored the charge dependence and asymmetry effects in ^{3}H(γ, n) and ^{3}He(γ, p). Agreement with the twobody photodisintegration (reaction a) data of (1981FA03) is fair, but some details are incorrectly predicted, e.g. the ^{3}H(γ, n) cross section is underestimated at energies below the peak. Electric dipole transitions are calculated by (1977MY01, 1979MA03) and comparisons with sum rules are discussed. A recent calculation by (1977VO11, 1981VO07) for ^{3}H(γ, n) and ^{3}H(γ, p) total cross sections used realistic NN potentials and the method of hyperspherical functions with an interpolation approach. The energy range considered was 10  8 MeV and the agreement with experiment was satisfactory over this range, but see (1981FA03) for a detailed comparison. A discussion of the integrated cross sections and moments of Table 3.7 (in PDF or PS) and comparison with calculated sum rules is given in (1981FA03). As noted there the value for the ^{3}He integrated total photodisintegration cross section σ(int) is 28.2 ± 2.8 MeV mb at 30 MeV (obtained by combining data in Table 3.7 (in PDF or PS) with published data for ^{3}He(γ, p) (1975FI08)). This comparable to the corredponding ^{3}H value of 29.0 ± 3.0 MeV mb.These values are about 40% of the strength predicted by (1978DR02) for the entire threebody photodisintegration cross section integrated up to the pion threshold. Agreement of the moments σ_{1} and σ_{2} with sum rules is poor (1981FA03) and raises questions about the adequacy of the calculations and the principle of charge symmetry.
Measurements of elastic and inelastic electron scattering cross sections up to q^{2} (fourmomentum squared) of 8 fm^{2} are summarized by (1975FI08). More recent measurements are listed in Table 3.8 (in PDF or PS) and 3.9 (in PDF or PS). The ^{3}H charge form factor was measured in the range 0.29 < q^{2} < 1 fm^{2} by (1982BE10). Both the charge and magnetic form factors were measured in the range 0.0477 < q^{2} < 2.96 fm^{2} by (1984BE46). Measurements of the charge form factor for 0.3 > q^{2} > 22.9 fm^{2} and the magnetic form factor for 3.1 > q^{2} > 31.3 fm^{2} were reported in (1985JU01). A very recent measurement of the isoscalar and isovector form factors for ^{3}H and ^{3}He for momentum transfer 0.09 > q^{2} > 8.4 is reported in (1987BE30). Charge and magnetic radii are quoted by (1984BE46) as r^{c}_{rms} = 1.63 ± 0.03 fm and r^{m}_{rms} = 1.72 ± 0.06 fm; by (1985JU01) as r^{c}_{rms} = 1.76 ± 0.04 fm and r^{m}_{rms} = 1.72 ± 0.02 fm; and by (1986MA2A) as r^{c}_{rms} = 1.81 ± 0.05 fm and r^{m}_{rms} = 1.80 ± 0.09 fm. Theoretical calculations for thr charge form factor of ^{3}H and ^{3}He were reviewed in (1977CI2A), who came to the conclusion that it is inadequate to use nonrelativistic wave functions resulting from conventional models of the twobody potentials, and to include only nucleon degrees of freedom in the electronnucleus interaction Hamiltonian. A variational calculation with correlated basis functions was carried out for ^{3}H and compared with the results of other methods by (1985CI04). The effect of manybody exchange currents on trinucleon charge form factors is to increase the height of the second maximum hence reducing the discrepancy between calculated and empirical values (1977RI15). The effects of the experimental uncertainty of the neutron charge form factor on the charge form factors of ^{3}H and ^{3}He have been investigated and are seen to be sizable (1975BR22). Analysis of parityviolating asymmetries in elastic electronnucleus scattering was made by (1981FI05). The effect of meson exchange currents on the charge and magnetic form factors of ^{3}H was investigated by (1975BA08, 1976HA33, 1976KL02, 1977HA03, 1977RI15, 1979GI08, 1981FR15, 1982HA09, 1983BE08, 1983DR12, 1983HA04, 1984MA26). Inclusion of meson exchange currents considerably improves the impulse approximation fits to the experimental data. See also the work of (1985TO21) on the calculation of trinucleon magnetic moments. The effect on the charge and magnetic form factors of clustering in the ^{3}H nucleus was investigated by (1976TA06). Recent calculations reported in (1987ST09) include singleΔ isobar admixtures in the threenucleon wave function. A discussion of electromagnetic form factors of ^{3}H is included in the review of trinucleon properties by (1984FR16) and in the very recent experimental paper of (1987BE30). The upper limit of the probability of the interior sixquark compound states in ^{3}H and ^{3}He was calculated from the electron scattering data by (1985KO02), and the effect of such states on magnetic moments was investigated by (1984KA25). However (1985KI12) find the method used to be unreliable and conclude that the mesonexchange current efects dominate those of the sixquark compound states. No new experimental data have been published on reactions (b) since the compilation of (1975FI08), though a preliminary experiment has been reported by (1984FR2B), however, recent data on reaction (c) at 500, 560 MeV is reported in (1986BA17). A calculation of threshold twobody electrodisintegration of ^{3}H and ^{3}He within the context of exact threebody theory is reported in (1977HE22). A theory for calculating spectral functions and angular distrobutions for electrodisintegration processes on ^{3}H and ^{3}He has been proposed by (1978CI2A, 1979CI2A). In (1979CI2A) a calculation was made of the quasielastic peak of ^{3}H and ^{3}He obtained by incoherently summing the cross sections for the twobody and threebody electrodisintegration processes, after integrating over the energy and the direction of the ejected nucleon. A variational threebody wave function is used in the calculations. The importance of final state interactions is pointed out.
Only a few experiments involving pions on tritium targets have been performed. See the review of (1978NE2B) which notes elastic scattering measurements at incident energies 132  187 MeV at angles 75°  135°, 25°  60°. Measurements of the charge exchange reaction (b) at E_{π} = 132  148 MeV and angles 100°  150° and of the capture reaction (c) at 132  148 MeV and angles 105°  130° are also listed. More recent measurements of pion charge exchange on tritium were carried out by (1980GL01) at incident pion momenta of 232 and 252 MeV/c and compared to theoretical predictions. Chargeindependence bounds were determined from elastic scattering cross sections.
Recent reviews of light hypernuclei are found in (1984BO2B, 1984SH07, 1986BO1E, 1986GI2B, 1987SH1H, ). For earlier work see the review of (1975GA2A), and see (1973JU2A, 1977RO04) and references listed in (1975FI08). Little new experimental information on ^{3}_{Λ}H has been published since the previous compilation (1975FI08). As noted there the ground state spin is J = 1/2, and some evidence exists for a J = 3/2 excited state. The binding energy of ^{3}_{Λ}H was measured (1973JU2A) to be 0.13 ± 0.05 MeV. Results of several measurements of the lifetime of ^{3}_{Λ}H are given in (1975FI08). A more recent measurement described in (1973KE2A) gives τ(^{3}_{Λ}H) = (2.47 + 0.62, 0.41) × 10^{10} s for the mean lifetime. Theoretical calculations of the ^{3}_{Λ}H binding energy are discussed in (1973DA2A, 1974BH2A, 1974GI2A, 1977KO2A, 1977RO04, 1980GE1X, 1980GI2A, 1980OG2A, 1980VE2B, 1981KO2D, 1981TO2B, 1982VE2A, 1983HA2B, 1984BO2B, 1984SH07, 1984SU2B, 1986BO1E, 1986GI2B, 1987SH1H). Calculations of the lifetime and twobody decay mode branching ratios are presented in (1979MA2B). Polarization effects are discussed in (1985LY2A), and a method of calculating the ^{3}_{Λ}H magnetic moment is presented in (1985NA2A). Considerations of chargesymmetry breaking are discussed in (1977GI2B, 1977RO2B).
